# What architectural tricks can I use to add a hidden floor to a building?

The Illuminati, Men In Black, NWO, Lizard people and various other secret organisations exist in my world, and for some reason (probably related to plot) they all favour having ‘hidden’ floors in tower blocks. The basic idea is that you walk into an elevator with X floors, punch the right buttons and end up on the X+1th floor (where the secret headquarters are).

Of course, it would miss the point of being in a high rise building if the headquarters didn’t have windows, but it would also miss the point of being secret if a third party could spot the fact there was a secret floor by simply counting the exterior windows.

What architectural tricks can be used to hide the fact that a whole extra floor exists from the outside of the building?

Restrictions

1: There must be one more floor than is recorded (architectural plans can be altered after construction), so no disguising the Secret Headquarters as another company.

2: All floors must have natural lighting and a view out over the city. Bonus points if you can achieve natural ventilation as well.

3: The number of floors visible from street level/adjacent buildings must match the number of floors recorded. More sophisticated methods of surveillance (thermal imaging, satellites etc) are not considered here.

4: No skyscrapers/all glass construction. Secret societies have a certain brooding monolith aesthetic that needs to be upheld.

The best answer is the simplest, most plausible way to hide the extra floor. As always real world examples/tricks that are already in use are encouraged.

• @L.Dutch: Various kinds of window, skylights, atrium arrangements, oversized architectural embellishments, light pipes and rooftop screens. None of them can achieve exactly what I want. The biggest constraint is achieving a view over the city while still being suitably masked from adjacent buildings. Dec 31 '20 at 10:53
• Closest I’ve got is an atrium inside the building with mirrors to bring in an outside view, but then it’s less a view over the city and more a couple of small ‘windows’ that look out over the roof of the building. Dec 31 '20 at 10:55
• Examples in fiction: Being John Malkovich, Grey 17 is Missing (from Babylon 5). Dec 31 '20 at 19:42
• Pretty much any building taller than about 10 stories has "utility" floors, sometimes full-height and sometimes only 5 feet tall or so. These are skipped by most elevator systems. And don't forget that, in the US, it's tradition to skip the 13th floor, adding a little useful confusion to the mix. Jan 1 at 0:57
• does the ENTIRE floor need hidden? or can it be part? Jan 1 at 3:28

The easiest places to hide an extra level are the bottom and the top. You can have all the subbasements you like, though they come with neither a view nor natural light. Either of these can get written off as maintenance levels.

You've excluded full glass towers, but dark glass towers hide their floor count better than clear glass, also having a dark tower looming over your area does carry that brooding monolith aesthetic that you're after. (They clad my local one in something more friendly recently).

The US is famous for not having floor 13 in many buildings, so that's vacant for your uses. People will see 80 on the lift, they'll count 80 and be happy.

Speaking of counting 80, the off-by-one error is your friend here. US buildings count from 1, Britain counts from Ground (0), if you put the building on enough of a slope you can have LG, G and UG before you start numbering them and the count on each side differs anyway.

Buildings of irregular shape have issues with window cleaning. Images of the Shard in London show robot arms sticking out at various levels for the window cleaners. You could close entire floors off as maintenance levels for heating/aircon/window cleaning without the day to day occupants blinking. Even easier is rather than having entire secret floors, you close off half a level.

Double height auditoriums also give you an excuse to close off the rest of the upper level of a pair. They'll know there's one on a lower floor and accept a level being missing above it.

You have so many secret organisations going that it's probably easiest to put apparent high ceilings on the outside, lower ceilings on the inside and have a secret floor on every second storey.

• Floor 13 is a clever idea. Refuge in audacity. And a little bit of foreshadowing that the secret organization is a little bit evil. Dec 31 '20 at 11:27
• Lecture halls, cinemas, theatres... all can house Secret Societies. That’s genius. Given that this is kind of a parody world and many of my secret societies have members/infiltrators/double-triple-pentuple-crossers in them it would make perfect sense to have a secret society headquarters society that just rents out hidden spaces in cinema complexes, providing many opportunities for hilarity to ensue. Dec 31 '20 at 11:33
• I’ve seen double sun-basements get natural light by use of mirrors and lenses. It’s amazing what enough money will buy in architecture. I was 30ft underground and couldn’t tell it wasn’t a direct skylight.
– SRM
Dec 31 '20 at 12:55
• "Stairwell doors can only be locked on every fourth level." "Doors not allowing re-entry must include signage on the stair side indicating the location of the nearest door, in each direction of travel, which allows re-entry or exit." - you could always bride the AHJ, but if I'm standing on the landing for floor 13, I know it's there, also because the sign says 14 is above me and 12 is below. Lessons from a High Rise Fire securitymagazine.com Jan 1 at 8:34
• On the 13th floor front, it gets more chaotic when you introduce other lucky (and unlucky) numbers; in Chinese culture 8 is lucky and 4 is unlucky, so you can easily get floor numbers like 1,2,3,5,80,81,82,88 for an 8 storey building Jan 1 at 23:19

The rooftop

What everyone knows that on the roof of every modern building there are a couple other boxes of stuff and an assortment of pipes connected to them. The AC, the elevator motor, the brains of a solar cell array...

What everyone knows is that those are typically not visible from the floor level.

What everyone knows that those boxes don't count as floors even though they can are big enough to house humans, and have a means of being entered by humans. Sometimes it's a locked door on the top floor with a rickety staircase that leads there. Sometimes the elevator goes all the way up to the engine room. Most likely both.

What people don't realize just how much stuff you can hide in one of these cubbies. An elevator engine takes up a lot of space. Sometimes the engine room is in the basement, rather than on the roof. That doesn't mean it can't have a rooftop engine room anyways - devoid of the actual engine assembly, of course. Instead, there's a spacious conference hall, open windows (opaque from above, thanks to special lensed glass, and hidden by the roof lip from below), walls of TV screens, perhaps a "service" door (or a retractable wall, be it for extra secrecy or just fanciness) in case the leader wants to take a rooftop walk, what have you.

• Guh. I now feel dumb. Hiding it within the various utilities that typically go on the roof is pretty obvious with hindsight! Dec 31 '20 at 11:16
• Why only the roof? images.app.goo.gl/pLeChqPbjcivFViN6 Dec 31 '20 at 21:39
• As far as space at the top goes, you can also generally include a water tank (most famously used in the film Towering Inferno). Standard mains water pressure isn't enough to reach the top of a high-rise building. It would be remarkably easy to subdivide that space to make room for your secret hideaway. Jan 1 at 17:00
• Secret Speakeasy: Abandoned Water Tower Nightclub in NYC Jan 3 at 3:41

Does the floor really need to be 'hidden'? I used to work in an office tower where the entire fourth (I think) floor was dedicated to machinery. I don't know what kind of machinery they needed an entire floor for. I think it housed the climate control stuff, and perhaps server space. This floor didn't have windows, but there's no reason why such a floor couldn't have windows for architectural reasons. I didn't find that floor strange, and I never heard anyone else about it.

It's the second floor of the tower section, with the big grills for (presumably) the HVAC clearly visible.

Although now I'm thinking about it, it might have been just this hidden society floor, of course. This was a bank, so that would be the secret meetings of the elite to control society. Wait, that isn't actually secret...

Thinking some more, do you actually need a hidden floor? Any self resprecting big company has some top floors for the directors only, where the underlings are not allowed to come. Nobody questions that. Any self respecting secret organization will have one or more front companies readily available.

edit: I should've just wikipedia'd it of course. That page offers many ways to hide secret floors in plain sight:

• "As a rule of thumb, skyscrapers require a mechanical floor for every 10 tenant floors (10%) although this percentage can vary widely", so just vary that percentage, and use some extra "mechanical floors" for secret meetings.
• Some types of buildings require outrigger trusses connecting the building core to the external shell at regular intervals for structural stability. If those are more than what's actually needed for mechanical floor space, there's your secret society floor.
• Sometimes the mechanical floors are clustered together. If you pair the mechanical floors and give them a lower ceiling height, you can add a third floor, while making it look on the outside as if the mechanical floors have an extra high ceiling height. Apparently that's just normal.
• Most skyscrapers above a certain height will have numerous hidden floors dedicated to machinery, spaced out every N floors as needed. They're generally accessed via service elevators, so not exactly hidden, just not where most people would know to look for them. Dec 31 '20 at 19:39
• @darrelhoffman do you also know what kind of equipment is housed there? A floor sounds like too much just climate control. Dec 31 '20 at 22:52
• @JanKanis I found this guide to designing a hotel: hospitalitynet.org/file/152004649.pdf Extrapolating a bit, it looks like you'd want a full floor for a kitchen would be needed every 10 stories or so of guest rooms. I would guess laundry facilities would also need to be replicated.
– SRM
Jan 1 at 0:47
• Nine of the 110 stories of Sears' tower are devoted to mechanical, in groups of three. Jan 1 at 5:08
• @JanKanis It can be all kinds of things, A/C and heating, water heaters and pumps, laundry and kitchens as SRM mentioned, networking (cable, fiberoptic, etc.) and telephone equipment, electrical, elevator machinery (generally at the top, but not all elevators go all the way up so possibly at other locations as well), possibly also some office space/break rooms for the maintenance people that run all this machinery, etc. Jan 4 at 13:42

# Windows spanning floors

If you pay \$, you can have single panes of glass spanning floors. These do not have to all be the same height. Let’s say a side of building is 5 windows wide. On first floor, have windows that are 8’, 10’, 11’, 7’ and 9’. As you go up the building, shuffle the window sizes around randomly, just picking sizes so they add up to the same amount in each column when you reach the top. Anyone outside the building cannot count floors just by looking at glass. Inside the building, it gets hard to be sure whether a given floor outside has the right place inside.

The weakness will always be someone measuring the height of your building and dividing by X, where X is standard floor height, but you can cover that by cribbing a few inches from each floor over the height of a tall enough building.

### A pyramid of penthouses and tilted windows.

Have a big building as tall as you need, and then put penthouses in a pyramid at the top:

      +-------+
/       \
+-+       +-+
/           \
+-+           +-+
/               \
+-+               +-+
|                   |
|                   |


You cant see them from the ground. You can put "penthouse" in the elevator buttons and rent one of them out, all the other floors can be secret.

By tilting the windows upwards it can reflect the sky. That's a good way to make the interior of the penthouse very difficult to see and examine from anyone up high. It also makes it difficult count windows from close up.

A high atrium in the public penthouse could explain the additional height to the casual observer.

• Better yet, hire someone to pose as the penthouse tenant. That way, you don't have that one tenant questioning why their suite seems smaller than it should be. Jan 1 at 13:34
• @jpaugh Or simply have a few of the more important secret society members be the penthouse tenants. That way, you don't need to fake anything. And you are not letting anyone in on a secret who doesn't have high personal stakes in keeping the secret secret. Ideal would be business men who are doing most of their work away from office anyways. They would leave their penthouse in the morning to get to work, and come back in the evening like any other working person would. Jan 2 at 22:59
• If you are going to have fake tenants, why not simply create a company and have them tenant out hire out a whole, publicly-visible floor as a front? Jan 3 at 15:23

This may not be as hard as you think.

Most large buildings are divided into offices. With a little bit of cleverness, you should be able to arrange that some chunk of a floor is simply (but not obviously) inaccessible via normal means. Put such an area above and/or below a double-high floor. Make sure it's obvious that the double-high floor exists so no one questions why there isn't an elevator stop for it. Then tuck a small stairway or ladder right next to the elevator on the floor that "doesn't exist". This gives access to the otherwise inaccessible area of the floor you carved into bits.

Of course, in modern buildings, do you even need to bother to hide it? Just set up your building security so that only employees of an office can go to the floors of that office.

I'm reminded of an old puzzle: Robinson Crusoe found a regularly-perforated hatch cover, and by some clever sawing made it into a solid table.

I think that in practical terms, if every "floor" in a building actually comprised several areas of slightly different elevation with ramps or a few steps between them, and if the external windows were similarly irregular, it would be possible even using fairly old techniques to make the appearance of the building so confusing that nobody could work out what was in it.

Or there's always the Trump way...https://nymag.com/news/articles/shortlist/trump/index.htm

• In Reading UK the council offices are actually constructed like this, each of the four lifts In the lobby go to every fourth floor (1,5,9) (2,6,10) etc. Trying to find a photo, but it could easily be used to hide a floor like this. Jan 1 at 12:12

Another answer. Does the architectural codes require the windows to be organized in a regular pattern? If the building is in a style with lots of windows randomly spread over its outside walls there's no way to deduce the number of floors from the outside. In fact any regular pattern would also do as long as it is not bound to the floors.

The thumbnail image of this video might be an inspiration: https://www.youtube.com/watch?v=bkzhsnjZKAQ

Build plenty of utility floors. In reality part of each utility floor is your secret headquarters. When you make your plans the space that will become the secret space is occupied by stuff that's going to be easy enough to remove after construction is complete.

The inspectors will not see anything wrong during construction and since it's a utility floor people aren't going to be wandering around.

I don't believe it will work, though--they'll be located by traffic analysis.

• If we're hiding which building has the HQ, a sophisticated, well-funded adversary who knew there was a secret headquarters somewhere in the city, could potentially locate the building based on traffic patterns. If we're merely hiding which floor it's on, but the correct building is known to all allies and most enemies (but maybe not the public), then traffic analysis is less helpful. However, if I were running a secret organization, I would simply create a very popular, end-to-end secured chat client (like WhatsApp), and have all my minions work from home. Jan 1 at 14:19
• Or, if every relevant player knew where every other player kept their secret HQ, and all of them wanted to hide their existence from the public (only), then hiding the base would still be relevant. They would not oust each other for fear of retaliation; and they would have other security measures to hinder espionage. Jan 1 at 14:24

Just use British level numbering when building and for the plans and door numbering and elevator controls, and American level numbering only for floor number as displayed in the elevators and on the ground level. It will take DECADES before people notice the discrepancy, and locate your Evil Empire one floor up from the ground.

British Building:
fifth floor
fourth floor
third floor
second floor
first floor
ground floor

American building numbering, for THE SAME BUILDING
fifth floor
fourth floor
third floor
second floor
First floor == ground level floor

• There are deep amateur analyses online for a building in my city that will start construction next year. I can find blueprint tear downs for lots of major buildings in USA from amateurs. I think this discrepancy will be noticed pretty quick without some further obfuscation. Obsession of hobbyists makes a lot of secrets hard to hide these days. Look at the tracking of Air Force One derived from what flight numbers WERENT used!
– SRM
Dec 31 '20 at 18:07
• @srm The local Woolworths building paid taxes on a non-existent floor for... 37 years(1965-2001, i think), before the discrepancy was found. Caused by just this confusion.. But then, here we are an ex-british colony, using british naming conventions but american building codes. Dec 31 '20 at 20:46
• There are many such stories in the past... I think the Internet works against any new ones happening like that. But I could be wrong. And, honestly, maybe lots of people worked it out over 37 years, but no one told the IRS. :-)
– SRM
Jan 1 at 0:37
• @SRM's take is probably correct for a highly-anticipated or high-profile building. Whether the public is sufficiently large and sufficiently engaged to contain a subculture which performs amateur speculation of the plans is probably (somewhat) outside the control of the Illuminati, et al. :-) Jan 1 at 13:40

Here's what I would do: instead of having extra floors, just have the elevator open in both directions, then on one floor, using some magic key or whatever, it opens in the other direction where there's a hidden compartment or room. The nice thing is that that is much less conspicuous as it just looks like the elevator shaft is a little larger on one floor. In fact, hidden rooms aside, this would make a great "maintenance area" in any high rise, since it's easily reachable, yet out of sight as well as it has a nice security door by default.

• This is similar to my idea. I would add two things: 1) The extra elevator door must be very difficult to see. Maybe it is a full panel instead of splitting in two; and 2) The secret area could be much bigger than a single room, with an obfuscated layout of the various rooms. Most visitors and clients won't expect to have access to the entirety of a building or floor, anyway. Jan 1 at 14:01
• @jpaugh Plenty of real buildings have such secondary elevator doors. Just make it actually useful in the real basement (for big deliveries of stuff) with a sign on it that it only opens on that floor - I've even seen some elevators with a separate button for the back door on a particular floor - 1-Front, 1-Rear, 2, 3, 4. As long as 4-Rear only works with a special code/key, nobody will know it exists or think anything is wrong with the rear door. Jan 3 at 4:13

Take some lessons from the Bar Convent in York. They wanted to have a church (and a convent) in a time and place where they suffered persecution.

From the outside, it just looks like a regular flat-topped building:

At the top, on the inside, there is a domed chapel:

Observe the natural light from the large windows, and the natural ventilation. The key here is that the uppermost section is not adjacent to the street. A person looking upwards from the sidewalk can't see the centre of the roof, and by measuring the angles exactly the builders were able to make sure that no-one could actually see the dome.

For your secret society, then, the key may be to make the top level of the building a dome or pyramid so that it slopes away from the street and can't be seen by passersby.

This doesn't meet all of your requirements, but I would ask you to reconsider the need for people to be fooled about the number of floors. Rather, we can hide in plain sight by fooling people about the layout and contents of those floors:

1. For example, a huge luxury hotel with amazing luxury suites on the top three floors. Several different reclusive billionaires end up renting all of the suites on the top floor on a pretty-much permanent basis. You need to use a guest card to take the elevator to the top floors. HQ goes there. The recluses of course need their minions, who show up every day...

2. You could also do this with penthouses in a very exclusive condo building. In both cases, the floor isn't hidden, it's broken up into units in such a way that people don't think about it as a large office area.

3. If you insist that the floors be "hidden", I'd think you could still take the top N floors and build a conference center, theme park, or some kind of area that would have a large auditorium in it. There would be "fake windows" to keep the look of the outside even though the Auditorium is a black box with no windows. "Looking at the top floors, you can't even tell that there's a huge no-windows auditorium up there. The architects cleverly placed fake windows in the same pattern as windows below." A "four-story-tall" black box auditorium will be extremely hard to see in perspective. It could easily be three stories tall with fake catwalks, ducting, etc, and since it's all dark up there -- painted black actually -- with lights shining down to blind you, who would know there's an entire office floor up there?

4. Or have the top two floors be HVAC and surrounded with huge LED video displays that play ads, etc. What looks like a solid panel of LEDs from the street would actually just look like a grid from inside, and the brightness of the LEDs would keep people from having a clear view behind the lights. Have a super-efficient HVAC system that only occupies the core of those floors -- which you would immediately see if you managed to get to that floor on the elevators -- and the periphery, with it's nice view, would be yours.

In the last two cases, people would know that the floors are there in some sense, but you'd fool them as to the contents and from the inside it would be very difficult to tell any different. In addition, it would provide the excuse for a variety of different people showing up.

And ultimately, what better place to hide a super-secret HQ than in the brightest and most visible spot in the city? Hide in plain sight.

• The biggest reason I can think of to actually hide it is to avoid attacks by people who want to destroy the HQ and have the ability to bring down a whole building. The "security through obscurity" has value in that case. I do like the way you broaden the brainstorming, but I encourage answers that address the question asked rather than try to talk the asker into a different question. :-) I won't downvote, but no upvote from me... others have different evaluation criteria. :-)
– SRM
Jan 1 at 0:42
• @SRM Were I the asker, I'd want (some percentage of) answers to re-reframe the question in subtle ways like this. The OP can't perfectly articulate their needs without giving all of the context. And, it's always possible that the OPs rules as stated don't perfectly match their needs. Jan 1 at 13:52
• @Wayne One weakness of this approach is that someone with binoculars or a camera and appropriate lens filters could get a very cozy view of your uppermost floors. It really depends on what threat-model the OP has in mind, but I'd guess the building owners will need to secure against nation-states and other well-funded, sophisticated attackers. Jan 1 at 13:56
• Remove that floor from the lift/elevator control panel, so the lift goes from 20 to 22 but floor 22 is labelled as 21. May need to finangle the car speed to make it go faster across that gap too. And will have to have trusted elevator technicians.
Bonus points if you hide the floor between floors 9 and 10, and call it 9&3/4.
• Disassociate the external cladding pattern from the floors. You could have windows that span 2+ floors, but this might be noticed, so have windows that are only ~13% of a floor height, but 7~8 of them per floor. Would have a "wood-like texture" to the outside.

If you want to forgo hard science:

• add a "magic portal" in the back of the lift-car that can be accessed by a secret button, or by dialling a special number on the button panel.
• Use a TARDIS as your lift car, so the entire secret area is inside the "penthouse" liftcar.
• Bonus points if you can do it to the Hancock building and give everybody X-bracing in their windows to charge more money. Jan 1 at 13:57
• To cover 20ft in the time it normally takes to cover 10ft, you're asking to double the elevator speed. That would be very perceptible, and make the gap obvious. It might be more plausible to have the elevator go slightly, but perceptibly slower. That would explain the time gap, and most would assume it was a technical limitation of the elevator shaft design. If the gap seemed too long, most would assume the elevator was going even slower than they perceived. As for your elevator technicians, well they have to be in on it, too! Jan 1 at 14:08

Everyone can judge the size and volume of rectangular boxes; few people can judge the size and volume of curved shapes.

E.g. Saint Paul's Cathedral in London has an inner and an outer dome; the space in-between contains the actual load-bearing structure, the shape of which architect Christopher Wren thought was much less attractive than a spherical dome. When you visit the building, you can't really tell from looking at it that the interior and exterior dome aren't one and the same.

When you visit St. Paul's Cathedral, you can go up into the space between the domes. I'd recommend it to anyone visiting London.

• TIL St. Paul’s has a hidden attic. Jan 2 at 22:04

# Televisions on the real floors

During daylight hours, it is very hard (bordering on impossible in recent years) to tell the difference between a modern TV screen with smaller-than-human-perception pixels showing an outdoor scene and an actual window. Floor X+1 can have real windows while floor X has screens. Cameras outside the building provide the feeds.

You’ll have to make sure the real floor empties out before sunset — true black pixels are nearly impossible to get right, and your subterfuge will be uncovered.

If you have control over the design of the real floors, arranging furniture to keep people from pushing up against the glass can make the illusion even easier to maintain.

• What is a "real" floor? (Which floors are unreal?) Are you saying to move the secret headquarters underground with fake windows? Jan 1 at 13:46
• Real = public. Unreal = secret. @jpaugh
– SRM
Jan 1 at 19:22
• No, anyone can tell the difference between a TV screen and a window, no matter how perfect the imagery is: The TV screen will be flat, and it will be at the same distance as the "window" frame around it. As such, your stereo vision will correctly tell you that the next door building is just five meters away from you, exactly where the "window" is located. What's more, the building in the "window" won't move across the screen as you walk past. And you can't just say "oh, the screens have cameras that track your eyes" because any number of people may be watching the "window" at the same time. Jan 2 at 23:20
• @cmaster For landscape vistas, I’ve seen the effect done, and it seems to work. There’s not a lot of parallax drift for crossing a room for something that’s a block distance, in my observation.
– SRM
Jan 3 at 0:54
• @SRM Have you moved up close to the window? Have you seen how the part of the image right in front of you grew bigger while the parts at the corners get compressed due to the flat viewing angle? I'm dead certain that such a display could only fool me as long as I'm stationary and at least ten meters away from it. The moment you start to move, there's ample amounts of perspective distortion to be seen. (That said, my depth perception seems to be a bit more trained than that of other people: I use stereoscopic vision for distances up to 50m. Other people seem to ignore these fine details.) Jan 3 at 8:34

Build the building on a sea cliff, and hide the extra floor at the bottom of the building. You can still get plenty of light in. Have dark glass extend beyond the bottom of your extra floor, and it will be difficult to tell where the floors end and where it becomes your 'architectural feature' glass at the bottom. Add in that the sea stops people getting too close to see in, and it should keep your society hidden.

For extra confusion, number it from the first non-secret level as 'ground' with the main entrance on the nth floor, whatever the top of the cliff comes out as.

I think you could make good use of the 'maintenance floor' idea. For example, there is a state-owned office building in Richmond VA, the Monroe Building. From its Wikipedia entry: "137 meters (449 ft) and 29 floors. Only 25 of the floors, however, are actually occupyable as the top and middle two are maintenance floors." It looks just like a perfectly ordinary skyscraper (by Richmond standards), and it has what appears to be a band of some type wrapping around the middle couple of floors, the maintenance floors. Nothing suspicious or odd-looking about it, and no reason for anyone to enter them. You could use something like this to hide in plain sight.

Have several buildings connected to each other but they are all at slightly different heights. This causes things like floor 2 of one building to be at the same level as halfway between floors 3 and 4 of another. Your "hidden" floor is explained as you skipped a floor number in one building to keep the numbering more consistent between the different buildings. You can even say the floor does exist but its just structural, again to try and keep the floors in the different buildings more in sync.

Consider turning the space vertical.

What if one corner of each floor of the building was sectioned off from the rest of the floor on each level, to put space for a stairwell and office. People could enter through some fire stair door. Each floor wouldn't have much space, but it could be as tall as fit person could climb.

If the rest of the floor was divided diagonally into two different office spaces for two (or more) different tenants, each tenant might reasonably conclude the other had the corner space that they didn't.

With many of these proposals, I don't think it is the general public on the street or the other tenants in the building that are going to be the problem. It's the trades-people. The plumbers trying to trace the source of a leak. The fire inspectors staring at the plans, wondering why they haven't had access to some areas. The electrician wondering if they can run cables through the cavities. The junior at the architecture firm blagging to their friends about what they designed.

Instead of using clever architectural tricks like fake windows, you can instead hide the windows on the floor by large advertisements on the outside wall of the building. Any person who will try to count the number of floors will just see the advertisement, making it very difficult.