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Neighbour built loft conversion on top of the party wall. Now I have to join my loft conversion to his and the neighbour is not happy.

Advice needed.
My neighbour recently built his loft conversion on top of the party wall, but did not leave much space. Now I am in the process of doing my loft conversion and my builder advised me that since the neighbour did not leave any space, we might have to join our wall to their loft conversion side wall and not stepping his dormer wall away from the boundary. 
My builder says this is to make my loft conversion watertight. My builder wants to join my roof and neighbours roof and dormer walls together.
The neighbours builder has told me that my builder has to have insurance in place should there be any damage or leakage, due to the roof being joined together, is this true?
My builder has blamed the neighbours and his builder for building on top of the party wall. 
I am really puzzled, would appreciate it if someone can advise me on what can be done in this case.
I am on good terms with my neighbour and don't want to fall out with them but I don't want to step my loft conversion away from the party wall, as I would be losing too much space.
Your advice would be appreciated.

Greg Holdsworth - 28/07/2022
Side Dormer loft Conversion built on party wallloft conversion built on party wallconstruct a loft conversion on a party wall
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You either may join to theirs.. or have a metre or more gap between the 2 dormers, not ideal but might save a lot of hassle and liability... if you do any work to join to theirs... their builder is out of the loop liability wise and everything is on you and your builder
Ken Cloke
I was under the impression you had to have min 300mm from edge of roof to the dorma wall ?? (In this case 300mm from fire brake in loft?
Colin Morton
Colin Morton I thought that to, and did they seek a party wall agreement when building on the party wall! I would have thought the party wall would have to have been built up with brick work if conversion like this.
Colin Harrison
You can build on your side of the boundary tight up in theory on extensions, but people are best leaving enough for footings and fascia so always best to come away from the boundary. We always start our trench 150mm away from a boundary or garden wall to save any hassle. This should be the same for loft conversions.
Rick Hodge
Most probably your builder will have insurance anyway,
But that is public liability and has nothing to do with leaks in the future. Your neighbour has built their wall on party wall
You don’t have much choice but to join and construct as one roof and God willing
It will not leak.
Faisal Khojah
Regardless of the other comments. You are where you are, you can’t change that so stop worrying. Join the dormers together. Make sure there is a cement board between for fire break. Your insurance and your neighbours insurance will be adequate. Your builder is guaranteeing his work so any issues (which I am sure there won’t be) he will address. Us builders don’t like clients claiming on our insurance We prefer to go sort any issue out. In ten years I have never had a single case of what you are worried about.
Dom Hearne
They are not meant to built on the party wall, how did they can get close to it? it’s kind of one of those things whoever does first may make it a little difficult for the other side. What the builders we use normally work with do is build the structure internally on your side, fireboard the side of your cheek and join the roofs and continue your tiles to meet your neighbour. In effect, you will have a void in between, so not directly connected and space between but it’s water right over the party wall. Ensure you have your neighbours approval for this, it’s better for them as well if you don’t do this they would also have trouble maintaining their side cheek.
Make sure you fire board superlux or similar, your neighbour should really have done this as well even without you having a dormer as it’s within 1m.
Ashley Manoff
There is no issue to build an adjoining wall as long they have the party wall agreement signed by you and your neighbour. If you know nothing about it at this stage, it means your neighbour/ builder hasn’t got his paperwork in place. You could even ask to move dormer back. Knowing this, gives you more leverage with your neighbour. However, if you are on good terms, speak with the architect about fireproofing and joining into the existing wall. You and the neighbours builders should discuss roofing detailing to see if they both agree on the method.
Mantas Prikockas
It’s fine to build on the party wall not much you can do about it now other than join them up. Look on the bright side, it saves your builder building 1 wall and tiling it all in although technically your neighbour could ask for financial contribution as the middle wall is now a party wall.
Tim Wilmshurst
Looks to me like they're not at the boundary, I'd say the halfway point between both your windows runs up to about 150/200mm wide of their dormer
Wouldn't this mean you either have to build yours over onto their side of the roof, meaning getting a party wall survey yourself? Or build so far enough away as is necessary on your side?
Bradley Latham-Holt

have a look here. It is a good source of what the process should be.
Stephen Whyte
Get an architect or engineer to have a look at it, it does look like it’s built on the dividend wall, which shouldn’t be unknown to you. If it is the case I would have my info together and have a chat with the neighbour.. they will have to work with you as by the looks of it a mistake has been made and it’s a mistake that’s making things an awkward and more costly on you now to do your conversion, possibly even could have planning implications that might have to be addressed before you start
Ken Cloke
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