For most homes in Keynsham, yes, and the paperwork is the ordinary kind. The town carries 87 listed buildings, which sounds like a lot until you set it against the thousands of houses standing around them. There is a single conservation area, covering the historic core. Two scheduled monuments sit in the wider area, and they are not houses anyone is extending. The short version is that unless you have bought something old or central, the rules are the national default rather than a local trap.
The 87 listed buildings are where the freedom narrows to almost nothing. Only two carry the top Grade One protection and three more the Grade Two-star tier, with the remaining eighty-odd at Grade Two, the everyday grade for a building of some historic interest. If your house is one of them, listing applies inside and out: changing a window, a fireplace, an internal wall or the render can need consent, and doing the work without it is an offence rather than a slap on the wrist. The consolation is that you would know in advance. These are a small, named set of buildings, not a hazard lurking under every roof in the town.
The single conservation area, wrapped around the older heart of Keynsham, is the softer constraint that catches more people out. Inside its boundary the council keeps a closer eye on anything that changes the look of the street. Some alterations that would be automatic elsewhere need a formal nod, the usual permitted-development freedoms can be trimmed back, and the trees get their own protection. It is not a ban on extending. It is a slower, fussier version of the same process, pointed at the front and the visible far more than the back and the private.
Everywhere else, which is the large majority of the town, the usual rules apply. A rear extension, a loft conversion or a modest addition inside the permitted-development limits can often go ahead without a full planning application, and where consent is needed this is not a town that fights every request for sport. The honest first move on any particular house is to check its listing status and whether it falls inside the conservation area before you fall for the floor plan. Those two facts settle almost everything about how free your hands are.
Extending in Keynsham is straightforward for most owners and genuinely restricted for a few. Buy an ordinary house on an ordinary street and the town stays out of your way. Buy a listed one, or something inside the conservation area, and you are building to someone else's rules. Middling freedom overall, and wholly predictable once you know which of those two houses you are holding the keys to.