Where to hide 200 m² of a low noise, low dirt, low profile operation for 30 years? Let's go through some possibilities.
In plain sight
200 square meters seems like a lot, but it is pretty much a rectangle of 20 x 10 m. Or in other words: a typical large home with a small garden. So, as it is not generating any extraordinary products, the first solution is easy: Hide it in plain sight. Some of the options you should consider, as long as the operation itself can run a very low profile and the paper trail between HQ and operations is nonexistant. Because there are laws that prevent random searches of private property without a warrent, these work. Water and power even come with them!
Renting office space for long term or even buying it might not be cheap, but place a name like "X&Y Consultants" on the door, hire a receptionist and have them take calls, telling the people "Sorry, there are no free spaces in the schedule of Mister X or Y for the next year... shall I put you on the waiting list?" or something. Anyway, who but the IRS checks consultant companies anyway? Just pay the tax and you are totally hidden in plain sight!
When we could have an office, why not just rent or buy some flats adjacent to each other in a nondescript appartment building and removing the walls between them, adding extra stairs and such till you are at 200m²? Nobody knows their neighbor in those buildings. Mount something in front of the windows to simulate people living there, never bother to change the nameplates on the bells and nobody will even notice. Just pay the bills from different funds and if somebody asks you who those immaginary neighbors are, say you think they all work odd shifts or something. In the run down "Banlieus" nobody cares who is their neighbor anyways, unless they can be blamed for something.
Fancy Skyscraper Anonymity
Well, maybe we want it more fancy than owning a ton of run down flats. Why not take the same idea and go fancy? Rent a penthouse or floor in an appartment building - and pay the porter extra to not be on the name plate. You are just that excentric guy living on the 10th floor, 200m² with perfect view on the Hudson or something. Expensive, but as long as you pay your bills, nobody will be the wiser.
It's all legit!
What kind of equipment does your HQ need? Often times, equipment can be used for multiple purposes at once, so find a solution how it can be done with what is in your lair! Even if they search, they shall not find anything but what they expect.
- A server farm? Easy, just BE a server farm, just one quite a bit larger with space dedicated for your HQ.
- You need a war room with spinning globe, huge TV screen and a fancy collection on the walls? Become a broker and have all this fancy equipment double as your screen for stock exchanges and show off how rich you are!
- So many different equipment that you can't keep track of it and so many goodies you want to stock? Be the R&D (in your own company, run by a straw puppet)! Nobody expects the R&D to plot world domination. It's allways the boss of the company, and those never work in R&D, right?
While we just had an official adress earlier, used to hide the operation, you might want some kind of a mix between Plain sight facade and actually hiding the equipment behind a false wall or space. We don't need to start precautions to hide our paper trail then, even welcoming a search of the open parts of the HQ. We need more than 200 m² then though - but power and water can be siphoned from the facade.
In the suburbs, houses can get large. Buying place where gardens are long and wide enough allows you to constuct your secret lair under the floorboards of a normal family life! Construction is a bit tricky, but it is not too uncommon to live in one space for a long time. Just hide the entrance to your lare under your model train landscape, and nobody is the wiser. Just appease the neighbors with an occasional BBQ and they will even vouch that you are clearly not Mister X, even if you plan world domination under the floor.
Money is not an issiue? Let's go BIG! I mean not like 500m², go fully large! Not just a fancy house in the suburbs, buy a whole factory, including workforce and produce whatever it produced all the time anyway. Schedule some expansion to the factory complex, and build your secret lair into the floor, then have the plans on file altered. That cellar? Was never built. Every morning the workers come, do their buisiness, secretly funding your plans, go... and under the floor? You plot and tinker on your secret world domination devices.
Famland is large, cheap, out of the towns... and farmers are crazy anyway, don't you think? In many rural areas people know each other well, and they know when Tom the sherrif comes for a coffee on his tour. Or when Mike Policeman has his patrol on the Highway. Again, keep up the facade well and nobody will be the wiser. And you can hide all the rubble and earth from digging your lair under the barn on or between the fields.
Out in the Blue
We can hide in plain sight with easy access, we can errect a false facade to hide better. But then again, we can go where people don't thread likely.
Can you have construction debris disappear with enough money? National park for the Rescue! Make sure to hide the entrance well, and the rangers will have a very hard time to accidently stumble over your lair - and since construction is prohimited in a national park usually, nobody will even think about looking for an underground construction there. Even better, if you can bribe the rangers to look the other way when you haul in a new batch of equipment. You better get autonomous in power and water means though, as you will have to drill your own water supply and sewage dump, and also some means of getting power. Geothermic power might be a solution.
What happens with an oil rig after it has run out of oil? Usually, they are deconstructed. But some are kept for other reasons - like an excentric millionair who might turn it into his private resort maybe? So, with enough money, an empty rig is available. What's even better is, some are out of the 12/24 miles zone of any country, and thus in international waters. What does that mean? You are out of jurisdiction of the country! Yes, that's right: Police and FBI can't go and arrest you on your oilrig out on the ocean. Why? Because a floating offshore oilrig is in the eyes of the law a ship (unlike one affixed to the seabed, which is an artificial island), and thus it is in the coutnry of the flag it flies. Just fly the flag of a country that is very unlikely to give a damn and your HQ is safe, even if they can see you building up a world domination device. Even better: if they try to stop you it is an act of agression against a foreign state without a declaration of war.
If you have mounts to the seabed and are further away than 200 sea miles from the shore, you are an artificial island wich belongs to the state that ordered its errection, which means it is part of the country you choose. For closer to the shore installations, it gets a bit more tricky, as construction of artificial islands there makes them part of the closest country (see Artificial Islands in the Law of the Sea).
Water purification can be powered with wind and tidal power, you just need to schedule monthly supply missions per helicoper.