I purchased a detached bungalow (first time buyer) with a garage with the aim of significant renovation work, including a garage conversion, creating a kitchen behind this and 2 further rooms at the back of the house. Although it is a detached house, it is very close to the neighbour - in fact the garage wall (my own wall) is attached to the neighbour’s wall. Due to the style of the build, their roof overhangs (over the garage) significantly which made identifying the boundary a bit difficult (not clear in deeds either).

Long story short, they were not happy with my plan and objected to every stage. Despite this I was successfully able to get planning permission and building regs approval to start work.

One of the concerns the neighbours had was the proximity of the build and the effect this may have on their foundations. They invoked their rights under the party wall act (which I honestly knew nothing about). I therefore paid over £700 to get the party wall act, which I am obligated to do but also wanted to show I was willing to cooperate. I had to compromise on 250mm of space, which does not seem like much but for my plans is significant (it is a conversion to create a disabled-friendly room for my mum).

One of the conditions of the party wall, or request, was that we allow the neighbour to replace their fascias. I therefore halted work to allow this to happen. For some reason they did not put a downpipe at the back at that time and stated they were happy for me to proceed with the work.

The structure has now been almost complete. Due to the proximity of the houses, my building inspector (who is hired by the local council) advised a shared gulley to driain both roofs. I suggested this (as my roof is higher than theirs) but they had a second opinion and said for ease of selling in the future, they wanted this (drainage of each roof) separate. So we put our own fascias above theirs.

So the drainage for my roof is sorted. But the manhole is in my garden (it is a public sewer which we relocated eith severn trent permission). I asked when the neighbour would put the downpipe on so we can connect it all up and she wants to put it on my wall (as the space is tight and not easily possible in hers). Note if she had done this before my build went up she would have easily been able to attach to her wall. I explained I was not happy to do this as if maintenance was needed then she would need access. I was also not going to accept responsibility for this downpipe as it is is solely draining her roof now. The other option is to relocate in to her garden and create a gulley but I think she is not keen to do this due to cost.

She has therefore hired a surveyor and told me a solicitor will be writing to me.

Originally the downpipe had been at the back of my garage and had drained both my garage roof and her roof into the gulley which connected in to the public sewer. The deeds also state that she has the right to drain rainwater via my land because of the way the pipes are laid presumably.

I do not want to have to fork out a lot of money. Nor do I want the responsibility for her downpipe. She is arguing that as I have been the one wanting to build and forced the move of the pipe, I should replace it. However I would have done this had it still been a shared downpipe. It is now just her pipe though.

Do I have a valid argument? If yes what else do I need to do?

I have had multiple issues with this lady unfortunately so need to be sure everything I have done is legal and cannot be challenged wrt the build

Incomprehensible. OP needs to consult local, qualified Solicitor.

Gutters dont drain intoa public sewer ( well shouldnt) so there is one argument less.

Basically for the rest of it you have made a rod for your own back by agreeing or conceding so may points it wont be worth arguing about this one. However, you might want to reconsider the entire buld and ask the council buildings inspector to consider new drawings....