Build extension astride boundary but existing extension?

dannim12345

Our semi detached neighbour already has a rear extension which is (as far as I can tell) built just within their boundary. 

We are now looking to extend out (not quite as far) and the architect said to keep it within our boundary. The neighbour asked why we weren’t building up against theirs, I assumed we couldn’t.  However I’ve since found out you can build astride the boundary if the neighbour agrees which may mean the extensions would butt up against each other. However when searching this online most examples are were there is no extension next door but the neighbour could use this as the party wall if they did extend. 

Anybody came across this scenario? 

Thanks 

Doozergirl

There's not a great deal of point building another wall as you'd have to flash into theirs anyway or there's a trap for damp and debris. 

You may be better asking if you can turn their wall into the party wall and recompense them accordingly.  

dannim12345

Thanks!  Yes it would create a small-ish gaps between the two properties so not ideal.  Too wide to flash into I would say. 

They didn’t build the extension but was built maybe about 5 years ago. We haven’t moved in that long ago but get on well (so far) and not planning on actually doing the extension for a while. 

I might have to speak to a party wall surveyor, not because it will be a dispute but to ensure we aren’t creating a boundary issue.  

ChasingtheWelshdream

My mother-in-law’s house is a semi has a rear extension that mirrors the neighbour. For some strange reason there is 30cm gap between them. It serves no useful purpose but does cause damp and is impossible to maintain.  It seems crazy that the two neighbours at the time didn’t just extend the party wall. 

dannim12345

Yes, this is defiantly the issue that would be caused but us also extending within our boundary too. Maybe not such a small gap. 

ThisIsWeird

It's impressive that your neighbour is being so sensible. Joining the two together makes sense for all the reasons shown above. And - on the surface - it's surprising your archi is recommending against this.

But I guess it depends on just how much encroachment would be involved, and for that I guess you need to work out just where the neighb's wall sits in relation to the true boundary.

If the original semi is symmetrical, then the 'exact' boundary should surely be easy to determine - it should be exactly in the middle. So, how much over that line would you need to go in order to use their wall as a 'party'?

I have no idea, but I'm guessing there would be a figure where the encroachment could possibly become an issue for the neighbour, should they come to sell? Eg, if the potential buyers were bolshie and would find the idea of this 'loss' to be unacceptable. I just don't know.

What does seem pretty clear is that a small awkward gap between the two is to be avoided. So I guess the options are to 'encroach', leave a full access gap (what a waste), or to build right up to the boundary, and then agree with the neighbour a method of sealing off any small remaining gap with a narrow wall, adding vents, and bringing the roof covering over to flash together. 

I have no idea how nutty that last idea is, but it would remove the feeling that one neighb is living on the other one's land (for some future owners) - that one has 'lost' to the 'other'. 'Cos there is honestly now't as queer as folk.

dannim12345

Thank you going to have a good look today. There is a low-ish wall between the two houses which I assume was the original boundary wall and their extension is inside that so it’s not much of an encroachment, about 15cm inside their boundary is where the wall is 

ThisIsWeird

dannim12345 said:

Thank you going to have a good look today. There is a low-ish wall between the two houses which I assume was the original boundary wall and their extension is inside that so it’s not much of an encroachment, about 15cm inside their boundary is where the wall is 

So for them to maintain their wall, they'd need to come on to your land? And you'd have to build a similar distance inside your boundary and remove that separating wall in order to leave anything approaching a suitable maintenance gap for you both? What a waste!

scottishblondie

Could you build a new party wall between the 2 extensions to fill the gap?  Then there is no worries about encroachment claims later on, and no need for a silly sized gap.  Just a filler wall between your new extension and their old one.

My IL's neighbours recently had an extension done and for some bizarre reason the council insisted a gap had to be left, so now there is a daft 5cm gap between their 90's extension and the neighbours new one.  Total dirt and damp trap!

dannim12345

Thank you, good points. I would imagine connecting them could cause the issue of movement as the foundation and build won’t be the same so will be the councils reason for leaving a gap. 

Phil4432

dannim12345 said:

Thanks!  Yes it would create a small-ish gaps between the two properties so not ideal.  Too wide to flash into I would say. 

They didn’t build the extension but was built maybe about 5 years ago. We haven’t moved in that long ago but get on well (so far) and not planning on actually doing the extension for a while. 

I might have to speak to a party wall surveyor, not because it will be a dispute but to ensure we aren’t creating a boundary issue.  

Go the party wall route whatever you decide, you'll have to pay for it as you are the one building the extension.

But it prevents further issues with neighbors in future.