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!

I am not a property lawyer but I am sure someone will be along soon who is (although your point seems to make sense).

However, even if he could legally build on top of your extension, from a practical point of view, that would all depend on how yours was constructed surely? I mean for him to build on your extension, your walls would need to be strong enough to bear the weight of an upstairs extension Also, he would obviously not be able to build over a glass roof or one with any glass in it, which would restrict your plans now, and probably make them more expensive.

So I would have thought the answer has to be no, just practically?

That should be could restrict your plans, not would. IDK what your plans are, obviously :-)

No, he can't. In fact, the air space above the garden may actually be in the ownership of the Council. It will depend on what your lease says but it definitely would not be your upstairs neighbour's.
(Ex-commercial property solicitor)

Wouldn’t that make it a flying freehold? I though that would apply to both freeholds not a lease so I dont think you can.

It wouldn't be a flying freehold because the neighbour is leasehold.

That’s what I meant as to why it can’t be done just poorly expressed sorry ( Clearly not an ex commercial property solicitor ).

Thanks all, he's just asked in theory, it would mean extra cost in the structural engineers report and the build, I'd started to look into this and then suddenly realised that it is our land! We have to get landlords permission from the freeholder (council) to extend which is what I'm currently in the process of doing. I could ask them but they are pretty unhelpful.

Unless it says differently on the lease I would have thought it would be up to the freeholder to decide what extensions can or can't be built.

I’m probably going to sound really stupid but what about your roof?! Will he recompense you for what it costs to put a roof on?

I don’t think he’d be allowed to do it but I’d seek professional advice if I were you

This is my problem, he's just asked in theory, and I'm like no - unless you commit now and we apply together and do it all at once. Which he doesn't want to do. I was hoping to head him off with the fact that he / future owners can't build as we're building into our own land. I'm going to have to get a lawyer to look at the lease, more expense and delay, was really hoping to get my permission application in this week before DH goes on a 2 week work trip next week!

Oh and not to drip feed but I need his party wall agreement signed before I can apply! It's such a massive curve ball and I'm hoping to avoid a lengthy delay!

I have no specialist knowledge but have exactly the same set up you describe and am currently sitting in our extension. There is no way the flat above could build on it. Firstly, the foundations would have had to be built to withstand that- which they were not! Secondly we have a massive skylight in our roof! Thirdly, we had a green roof put on it! And most importantly in my mind, it is ours!! You can't just build on top of someone else's property!

What wiifitmama said. Just explain that the foundations of your single storey extension will not support two storeys. That should end any confusion.

Hi, we are in exactly the same position and out planning approval specifically stated no-one could build on it (but reason was for neighbour amenity so if they, the neighbours want to, that clause falls away?)and foundations specced only for single storey. However, they say that their structural engineer has looked at the calcs (available online in planning application) and says that they will support another storey! Do we now have to pay for another structural engineer to prove otherwise??! Not sure who's decision it is either about whether they can build there or not: ours, freeholder, planning authority? Sorry to hijack OP but would be interested in views too.

No prob @Chilver - we are just at the planning stage and I need his signed party wall agreement so have to look into it for him but hoping answer is no!

@Seahawk80 it's not your land though, it's the councils land. You are leasing it.
To my mind, the answer to your neighbour's question is "in theory yes, but we would both need to get permission and build it strong enough. If you want to do that let me know ASAP. We can share the costs of making it strong enough and getting freeholder and planning permission. Our architect is x person, you'd probably want to engage her, here is her number for a quote. Can you let me know by Sunday what you are thinking?"

My understanding is you own the land, but not the airspace. But as he is building on top of your extension, it's still on your land, as it's on top of a building on your land. It's not floating in the air like a helium balloon.

So the answer is no he can't do it, not unless you permit it, and I wouldn't, it could cause issues later.

Thanks everyone. I will update once I get an answer. I think I'm just going to say to him we're planning a single storey extension and there is also the legal aspect of building above our garden and you would need to look into this too, pease let me know if this is something that you would like to pursue at this time.

No, they could not build on top of your extension unless you put the foundations in to bear the weight of a two storey extension. The same way as you cannot build on top of a garage without those foundations.

I highly doubt it! What if he built on top of your extension and then a few years later you decided to knock it down!

Don’t have a flat roof, that will make it obvious he can’t build on it.

Thanks all, as it's not built yet, he's asking if it could be done in theory in the future, it's just such a massive curve ball and would mean redoing all the plans etc, I was hoping that legally it would be a simple no. I've checked our lease and the boundaries are very clear so I'll have an initial chat with the lawyer who did our conveyancing in the morning and take it from there.

Make sure you include lots of glass in the roof. In theory, you could build on top, but you wouldn't let them!

As you don't hold the freehold, in theory they could get permission from the freeholder, BUT to make a double storey extension you need different foundations etc so not that simple

Just a thought - if you get approval to do just your extension without any input or say from the upstairs neighbour, make sure your extension foundations are only deep enough for a single storey extension. That way there would be no chance for an upstairs neighbour to do the same, unless they wanted to demolish the downstairs one at the same time.