Yes, you can extend most houses in Wallingford, but the closer the house is to the medieval centre, the more people get a say in it. The old town is a conservation area. More than a hundred buildings are listed. The town's Saxon street plan is protected as a monument. Most homes outside the old centre were built after the war, and those are much easier to extend than the historic houses.
Wallingford has 148 listed buildings. Four are Grade I, 12 are Grade II* and 132 are Grade II. Listing protects the whole building, inside as well as out, so changing a fireplace, a staircase or a sash window needs listed building consent from South Oxfordshire District Council as well as any planning permission. Most of the listed buildings are Grade II, and they are ordinary houses on ordinary streets. Plenty of buyers only find out at the survey that the house is one of them.
The town has one conservation area, named Wallingford, and it covers the historic centre. Inside it, alterations that would normally go ahead without an application get more scrutiny, especially anything that faces the street, such as windows, roofs, front walls and cladding. The council's conservation area map shows which side of the boundary a house is on, and that is the thing to check first on any house in the centre.
Four scheduled monuments stand within the area, including Wallingford Castle, the town walls and the Saxon town itself. Scheduling is the strictest protection there is. For houses on or beside that ground, digging footings can mean dealing with archaeology as well as building control. In 1086 Wallingford was important enough to be listed in Domesday Book, and the protected ground is a result of how much was built here then.
People do alter their homes here. The planning records list 1,349 applications between April 2024 and June 2026, 595 of them in the last twelve months. Most of those will be owners doing much the same work you have in mind, so the council's planning officers will have dealt with it plenty of times before.
In practice it depends on the house. A post-war semi or detached house outside the conservation area usually has permitted development rights for modest rear extensions and loft work, subject to the usual size limits and to those rights not having been removed for that property. A flat has no such rights, so anything beyond internal work needs an application. A listed house needs consent for almost any change. A house in the conservation area needs more care with anything that faces the street. The same rear extension can need no application on a 1970s close and a full set of drawings two streets away inside the old town.
Wallingford is about average for freedom to extend. Most of the housing gives owners room to build, and the historic centre is far more tightly controlled.