I have had a quick look through the drawings and I can see that you are proposing to extend the party wall up both sides to form a rooftop extension and to extend the party wall back into the garden on both sides to form a single storey extension. These aspects will exercise rights under the Act as you have correctly surmised and I imagine that there may be other structural elements to the works which will be notifiable such as creating foundations possibly deeper than the existing and cutting pockets in the party walls for steels etc.

I should firstly advise that you do not have an automatic right to create a new party wall in the garden. I think it is the best solution when extending to propose to create a new party wall. The neighbour can in due course make use of it but would have to pay back half the construction costs at that time. This would be beneficial to all parties both in terms of maximising space and cost. However, if the neighbours do not consent to the construction of the party wall, the wall will have to be built entirely on your side of the boundary except projecting footings.

If you are proposing to commence works on 22 August, about now is appropriate timing for service of notices. The minimum statutory Notice period for Party Structure Notices is two months.

I am afraid that there are firms of Surveyors who use the Planning Application lists to send out touting emails and letters. This is not in my view the way to proceed and I very much doubt the neighbours would consider appointing them if they had to pay the fee. My suggestion in this instance would be to encourage the neighbours to follow the Agreed Surveyor route. I am quite happy and confident to act in that capacity. Ultimately, the Adjoining Owners may choose their own Surveyor and the reasonable costs incurred will be your responsibility.

I presume that you have or will be appointing an Engineer. I will need to see the structural proposals before serving Notice. Before I can put Awards in place I shall need details of the works which are proposed under the Act. In particular the sensitive areas will be breaking through the Adjoining Owner’s roof and producing a new weatherproof junction at that point and access into the adjoining gardens to construct the new party wall if indeed they consent. If they do not consent I will still be able to provide access under the Act but the wall will have to be entirely on your side except for any projecting foundations.

On the basis of the drawings you have sent and assuming there is one owner either side upon whom Notice has to be served I would be prepared to offer a fixed fee quotation in the sum of £1,750 plus VAT and out of pocket expenses to carry out all duties up to the point that Awards are put in place both sides. Hopefully very little input will be required after that apart from getting the Adjoining Owners’ Surveyors to discharge the Awards. That will be charged at my hourly rate which is £150 plus VAT or £90 plus VAT if my assistant deals with the matter.

You will be responsible for the fees incurred by the Adjoining Owners’ Surveyors. Typically the fees are likely to be in the region of £1,000 - £1,500 plus VAT per Award and that would include for the final inspection. Some central London practices could be more than. If the neighbour agrees to my selection as the Agreed Surveyor then there are no Adjoining Owner’s Surveyor’s fees to meet but I will have a certain amount of ongoing responsibility to visit the works to check on matters such as access, setting out the position of the party wall and checking off the Schedules of Condition at completion. Those additional duties could perhaps be a further £1,000 - £1,200 plus VAT.