Warwick District Council is the local planning authority. The district has 29 conservation areas, and four of them sit in the Leamington area alone: Royal Leamington Spa, Lillington Road North, Lillington Village and the Canalside. If you are anywhere near the centre of town, assume you are in one until you have checked.
Paint is a planning question here
Warwick District Council publishes separate guidance specifically on painting buildings in the Leamington Spa conservation area. Councils do not write documents about paint colour unless it matters, and in a town whose character is largely made of painted stucco terraces, it matters a great deal. If your building is in the conservation area, treat the finish and the colour as something to check rather than something to choose off a card, and check it before the scaffold goes up rather than after the first coat.
Listed buildings
Leamington has a lot of them, and listing covers the whole building inside and out, not just the frontage. Listed building consent is a separate matter from planning permission and doing work without it is a criminal offence rather than a paperwork problem. We ask about listing at the first visit, every time, because the answer changes the entire job.
Everything else
Out on the later housing the ordinary rules apply, and a single storey rear extension often falls within permitted development. Permitted development is not an exemption from Building Regulations, and a full plans application is still required. We do not make that application, but we arrange and meet the inspections that follow it and we see the work through to sign off.