Build at the same time

It’s possible that your neighbour in a terrace also has an extension in mind, so if you have similar goals, why not consider doing the work at the same time? You might be able to share an architect’s fees for a mirrored design as well as economise on building costs, including those for materials, and items like skips. If one of you has access to the front of the homes via a passageway, this can also be moneysaving. The other advantage isn’t financial, but it is valuable: you won’t be disturbing each other with building work.