Most of the standard extensions don’t need planning permission – enough to supply full plans application to the Local Authority Building Control. On average it will take about 3 weeks to get the building control approval. At the same time (or better even much earlier) the party wall notice should be served to the neighbours. Legally they have two months to consent or dissent. This applies when the proposed works will be carried out on a boundary line or close to it – most often you will need it. In case of dissent there will be a bit more paperwork, and it will take longer to get to the building works starting date.