Gap or no gap between single storey rear extensions of two semis?

[NoIdeasForWittyNickname]

We’re planning a rear single storey extension to our semi-detached house. The neighbours in the other half of the semi already have an extension and its wall is set about 15 cm from the boundary into their land. If we mirror this, we’ll end up with a very narrow gap between the two extensions which I hear is a potential maintenance disaster

I’ve had a quick Google and it looks the best option from structural point of view is to join both extensions together. Looks like this will require a party wall agreement or something like that. We seem to get along with the neighbours, so I hope this won’t be a huge issue - am I being too naïve?

Additionally, the neighbours have multiple cables running on the outside of that extension wall, so ideally they’d need access to those. Currently, we do let them into our garden if they need to tidy up or check the cables, but that’s just out of good will, as there’s nothing in our deeds about their right of access. But more importantly, there’s a drainpipe which collects rainwater from the roof and into a gully in the ground. Most likely in the past it was a joint one, as it’s right on the boundary line, but when the neighbours built the extension, they reconfigured their gutters and now it only collects water from our half of the roof (but it’s my guess as their extension was built before we moved into this house). If we want to build over that spot, would we have to relocate the drainage, or can we just seal it up? Is it prohibitively expensive and complicated? Would we need to get a solicitor to amend the deeds?

But wouldn’t that mean building your extension on 15cm onto their land? And them moving all their cables? I’m not sure anyone would go for that would they?

I wouldn’t agree to that if I was your neighbour - you’d be gaining 15cm of my land! Would you be offering to pay me for it?

1. You would need to pay for the 15cm encroachment into their land.
2. You will need to pay for your structural engineer and theirs. You may also need planning permission.
3. Assuming they agree what will you arrange with the cables?

Good point about the 15cm ingress, I thought of that too! We don't want just to grab their land! I was thinking more like staying within our boundary but backfilling the gap. Sort of making their wall thicker by adding more bricks to get up to the boundary on their side to meet our wall (well, maybe us actually doing this in course of our extension, or sharing the costs, with their agreement, of course)? And yes, the cables...
Just wondering about pros and cons from structural and legal point of view.

Leaving the gap is obviously easier. But if we mirror their arrangement, the resulting 30cm will be too narrow for maintenance and access. Anything extra will come at a cost of the internal space in our extension. What's the reasonable gap for maintenance access? The neighbour is quite slim but what if he gains weight in the future?

Thanks @GrumpyHoonMain, useful to know proper terminology (like encroachment), this will help my research. We'll definitely seek professional advice and talk to the neighbours. But maybe we should just discount that option from the outset and settle for the gap. Just feels unfair that because they beat us to the extension, we now have to dance around worrying about their access issues and the drain

Talk to the neighbour. You could build up to the boundary line, but get agreement to join roofs so water doesn't get into the gap. Then a blanking sheet (or brick wall) can be added to close off the gap in walls. Leave as a cavity. You don't want damp issues by debris building up in a narrow gap.
Cable management is your neighbours problem.
If the drain is your side of the boundary, just move it. If on the boundary you should get permission. Check nothing else like a sink drain joins it from your neighbours house.

Get any agreements in writing though. I'd be interested to hear what works from a solicitor.

I lived in a house with a small gap between rear extensions. I’d say make the gap large enough to get someone in to rescue the assorted animals that will undoubtedly get trapped in there. Or so they won’t get trapped!

Can't see a huge issue here with the wall to be honest. A party wall agreement which you'll need in any event and you agree to build astride the boundary, all done up in a nice legal bow of course. A 30 gap running the whole depth of the extensions will create a damp problem eventually as there isn't enough air flow.
More of an issue may be the roof lines and guttering. I assume you're measuring from the wall and that the gutters of your neighbour actually run up to the boundary limit? I don;t know why but I'd be more cautious of joining roofs and sharing guttering; then again it is already a semi so why not?
Preferred solution would be to leave a 1m gap but then why should you not enjoy the full width as your neighbours have?

Thank you all, very good points about animals getting stuck and potential dump issues! Ok, definitely need to talk to the neighbours. I'll be stalking them for an opportune moment this weekend

@Loofah01, interestingly, they don't have gutters on that side. The extension has a flat roof and its edge is pretty much flush with the wall. I've just looked closer from our bedroom window and noticed there's a little lip along that roof edge that sticks up about 3cm above the roof level, which stops water from simply spilling over and running down the wall. I think all their rainwater from the rear now drains towards the other side of the house.

@Rollercoaster1920 we're at a relatively early stage. We're also thinking about loft conversion, so most likely will end up going the formal PP route. We've had a conversation with an architect who sketched a few layout ideas but he hasn't picked up on the issue of the gap/drain If anything interesting transpires on the legal front, I'll post the update.

interestingly, they don't have gutters on that side. The extension has a flat roof and its edge is pretty much flush with the wall. I've just looked closer from our bedroom window and noticed there's a little lip along that roof edge that sticks up about 3cm above the roof level, which stops water from simply spilling over and running down the wall. I think all their rainwater from the rear now drains towards the other side of the house.

It’s called a parapet wall.

Thanks @Mamette

Wondering if you came up a solution here and how it is going? We have a nearly identical situation minus the pipes.

We also have this problem and have been wondering how to resolve as we can build to the boundary and don't want to loose space by leaving a big gap.

We agreed with the neighbour that they would extend their extension up to the new party wall, joining the rooves so it looks as one.

Roofs?

I’m not sure I follow. Did you build a party wall?

Yes.

And to follow up, did they pay you for the party wall usage?

Any update on what you did?! Same problem here too Haha! Our builders have started work this week on our extension, which will run parallel to our neighbours existing extension. Our neighbours have already been on the phone complaining that we aren't going to be leaving a big enough gap for maintenance, the gap will be 375mm. It's tight but I feel like it's enough as it was them who wouldn't agree to us enclosing on their existing wall...

We had a semi and a neighbour who had a conservatory put up close to the boundary. Damned annoying, as it was put up while we were at work, without him mentioning it first.....
When we had ours built next to it, we left enough space for a ladder to go in between should either of us need access to the upper house gutters or the conservatory ones. (By then we had different neighbours.) We agreed to remove the old fence between the two as neither of us had kids or pets, and it meant we could both get in easily to clean out gutters, etc. It can work if you're friendly.