Can my upstairs neighbour build on top of an extension into my private garden?

[Seahawk80]

I'm planning to do a small straightforward extension at the back of my ground floor flat. I'll try to put all the details relevant details below:
Victorian Terrace in London split into 2 flats
Both leasehold (freeholder is the local council)
Both privately owned
The flats are split ground floor and upper floor and we own the whole garden, upstairs have no access at all to the garden.

Our upstairs neighbour has been very reasonable about signing the party wall agreement but had a few questions including would it be possible to build on top of the extension in future. He has no plans to do so but wanted to know in case future buyers ask. I was looking into the practicalities but then thought surely he would t be able to as it is our land. If you lived next door to someone who had a ground floor extension you wouldn't ask to build on top it! Or is it not that simple? I've asked my architect who agrees with me but if anyone has any specialist knowledge please help!

Normally, the freeholder owns the walls and roof. This is why they are normally responsible for maintenance of these. I assume you pay a maintenance charge. If you build an extension, I assume you give the new walls and roof to the freeholder. Therefore I do wonder if it’s their decision about what is built on it? The lease gives you permission to live in the building but I’m assuming you don’t maintain the external features of it. I would get legal advice from an experienced property lawyer.

You would need to do foundations to suit the ground conditions. Anything less would be negligent and asking for trouble.

Don't know what the legal position is, but for him to be azble to do this, you would have to have different (more expensive) foundations etc. If he isn't prepared to pay for these NOW, you won't want to do it. So the answer is no.

Interested that you say you are using an Architect. Good for you. But are they actually an Architect, or are they a builder? I ask, because you're not allowed to call yourself an Architect unless you actually are one (and registered as such). My DH is a qualified Architect and gets very annoyed about this!