Mostly yes, because the heritage rules reach very little of Didcot. There are only a couple of dozen listed buildings. Three small conservation areas cover the older pockets. The neighbours file hundreds of planning applications a year. The real constraint is the ground, which is clay that swells and shrinks right across the town.
Didcot has 22 listed buildings, none of them Grade I, 1 Grade II* and 21 Grade II. Any alteration to a listed building, inside or out, needs listed building consent on top of any planning permission, and doing the work without it is a criminal offence. With only 22 in the town, the odds are good that the house you are looking at is not one of them. Didcot spent centuries as a small Berkshire village and only grew after the railway arrived, so most of it is too young for anyone to have thought of protecting it.
There are three conservation areas: Didcot Old, Didcot Northbourne and Didcot Station Road. Inside them, the council looks harder at anything that changes a building's outward appearance, and some work that would otherwise need no application may need one. South Oxfordshire District Council's conservation area maps show where each boundary runs, and a house on the edge of the old village around Lydalls Road and Manor Road is the kind worth checking. Outside these three areas, Ladygrove and Great Western Park face no heritage controls at all.
Between April 2024 and June 2026, 768 planning applications were made in Didcot, 362 of them in the most recent twelve months. That is a town busy adding rooms to houses that were built to a price, and the planning register shows the neighbours have tried most of the obvious ideas already.
The British Geological Survey puts shrink-swell ground at class 3 across the whole of Didcot, meaning clay that expands when wet and shrinks as it dries. That is a published hazard class, and it matters most for foundations. An extension here will usually need a structural engineer to set the foundation depth, particularly near trees, and building control will expect it. The drawing of the extension is the cheap part, and the money goes into the ground.
For most houses outside the three conservation areas and the 22 listed buildings, standard householder permitted development rights are likely to apply, so a modest rear extension or loft conversion may need no planning application at all. Some estates have had those rights restricted by conditions on their original planning permission, so the council's planning history for the specific house is the place to confirm what is allowed. The heritage system will mostly leave you alone, but the clay and building regulations will not.
Your freedom to extend in Didcot is middling. The planning side is fairly easy, and the clay under the town makes the building side harder.