The officer’s report on this one is nice and clear. The single-story rear extension is too big for a semi-detached house as it extends to 5.85m. For it to be permitted development the scheme needs to be pulled back to 4m from the rear wall to comply with this. It looks like no.20 already has a single-story addition on the northern boundary, so a planning application might be acceptable for this proposal, the applicant has to go through the full process.