The Great Wall of China can be seen from space!
Not quoting you, obviously, but it's a thing we've all heard a lot. We can see a load of things from space - with Google Earth I can see my house! It's good that you don't seem to be making that mistake, but on the off chance that was in your head, I just wanted to make sure we're on the same page.
I was a bit confused by your question though:
We can see the planet but we can't see them trough basic telescopes
I'll do my best to answer your question anyway.
I'd like to direct your attention to Mars. According to the link I've attached, many famous observers tried to learn about Mars with a standard advanced telescope of their time. They learned about how certain parts of Mars's orbit took it further away then it should've been, indicating some knowledge of a non-circular orbit and one that is not synchronous with Earth's orbit. They also started noticing Mars's polar ice caps expanding and contracting. Later on, they started noticing the massive dust storms we know Mars for (although they didn't know that that was what it was). If this is the tech your people have, they'll probably only notice this much. All of these observations took place by early 1800s.
By the mid-1800s, people were noticing features on Mars itself. People started making maps of the surface by this time. Not all of it was accurate, but what was very accurate was the rotation of Mars (which we figured out to be accurate within a tenth of a second). By the early 1900s we had maps of canals on Mars (we were taking photographs at this point), but later on we noticed other features, but almost a clear absence of canals.
Just by this level of tech, we can safely assume that without putting something much closer to Mars (like a satellite) we can't be too sure of buildings built by an alien race unless it's really obvious and changes the planet itself. Now, however, we can take pretty great images of Mars from Earth (no need for satellites), but our optical and digital zoom technology had to be refined a lot, and our best images are taken from out of our atmosphere to prevent distortion. Also, one reason we have such nice pics of Mars is because it has barely an atmosphere, so nothing to get in our way when we're looking at it from outside.
To look at your question again: you can't see this planet with a telescope, but you know it's there. Mars in our real world case could be spotted with the naked eye and observed with a good, old-school telescope. We still can't see anything on Mars though - until some nice pass-bys with various probes and satellites around Mars. That's despite having an advantage over your case. With your logic, the planet would need to blow up or something so we can notice it. I'll correct this answer if that's not what you meant.
EDIT: From comments and from clarification from the asker, I'd like amend my answer. Unless the structure the aliens were making on this planet were big enough to be noticed like dust storms, polar ice-cap expansion/contraction, or some other massive weather phenomenon, the odds of your people sitting on another planet being able to notice them is pretty slim. So, if they build a bunch of Empire State Buildings - no one would know. Now, if they had fast-growing cities or other megastructures under construction, they could be noticed, given that these construction projects weren't mistaken for weather activity.