Change (+ or -) in property or land values are not a material planning consideration and thus will not be taken into account.

Mud on the road would if it was to cause some danger but then the planning authority would likely just impose a condition requiring adequate wheel washing facilities for all vehicle leaving the site. In building anything there is always noise, dust and disruption and for that reason the planning process tends not to get involved in such matters except in extreme cases where its likely amenity would be severely harmed for an extended period of time - even then I can't recall permission ever being refused on the grounds of such construction disruption alone. It's more likely conditions would be imposed to govern the construction process, methods and times.

The planning authority is statutorily required to determine all planning applications in accordance with the development plan unless material considerations indicate otherwise. The development plan is usually comprised of a local and regional based plan with the local plan being more relevant to matters of single house developments and extensions. The policies in local plans will vary from council to council so I cannot offer specific comments on this development relative to the policies of Lewisham.

However, in general I see a 3-story house that is being sympathetically extended in a manner that captures an opportunity to create a pleasing symmetry. The existing height and period features of the house appear to be retained and the extension doesn't appear to make it overly disproportionate or bulky. Furthermore, the basement extension has little impact on the external appearance of the property. It would appear that the building is arranged in a row with it's neighbouring properties (16...and I assume 12) and thus the extension would not give rise to harmful overlooking or overshadowing. In short I'd be surprised if this type of extension, which in terms of its size and design to compliment the existing building is very common, would be refused. Matters of parking and plot ratio's may be a consideration though but these are very council specific.

I realize that as this is happening next door to you it's a lot harder to be so accepting. It no doubt appears as a monstrosity that will ruin your life and the value of your home. However, I'm simply stating as an outsider that my first impression is of a fairly sympathetic proposal that doesn't appear to throw up too many planning issues. The best you can do is to put your concerns with the proposal in writing and submit that representation to the Council before the cut off date. Try to be as clear and concise as possible in your letter and don't get too hung up on issues of value, disruption, etc as these things are not going to be considered.