You can extend a house in Cirencester, but across much of the town you will need permission from people who care a lot about stone. There are hundreds of listed buildings. There are four conservation areas. Almost the whole town is built on clay that swells and shrinks. Outside the old core, extending is ordinary suburban planning. Inside it, every change to the house needs signing off.
Historic England lists 358 buildings in and around the town. Three are Grade I, 33 are Grade II* and 322 are Grade II. Listing covers the whole building, inside and out, so new windows, a moved staircase or a knocked-through wall can all need listed building consent from Cotswold District Council, as well as planning permission for anything bigger. Grade II covers most of them. Even so, a Grade II house will not get a glass box on the back without a long correspondence about it first.
The town has four conservation areas: Cirencester Town Centre, Cirencester Park, Cirencester South, and Cirencester Gloucester Street and River Walk. Inside them, the council looks harder at anything visible from the street, including roofs, windows, materials and side extensions. Some of the work an ordinary house could do without asking needs an application here. In the early twentieth century, the Arts and Crafts architect Norman Jewson practised in the town under the patronage of the Bathurst family. The standard a new extension in the Town Centre area gets judged against is roughly the standard he set.
Seven scheduled monuments are close by, Corinium Roman town among them. Scheduling is the strictest protection there is, and it mainly matters to anyone whose land touches one.
The volume of applications shows people do get things done. Between April 2024 and June 2026, 2,829 planning applications were made across the area, 1,311 of them in the last twelve months. Cirencester residents alter their homes all the time. Whatever you have in mind, a planning officer at Cotswold District Council has very likely seen something like it before, and has a view on the brick.
The British Geological Survey's ground-stability mapping puts the area's shrink-swell hazard at class 3 at its highest, and about 97% of the area reaches that class. Shrink-swell clay expands when wet and shrinks when dry, so new foundations may need to go deeper than a builder's standard quote assumes, especially near trees. That is a matter for a structural engineer's report and a sensible foundation design. It is no reason to drop the idea, and it costs much less when it is planned for from the start.
For a post-war semi in Stratton or a newer house towards The Beeches, a modest rear extension may well fall under permitted development. That freedom shrinks inside a conservation area and does not apply to the listed parts of a listed building. Ask Cotswold District Council in writing whether the house is listed and whether it is in a conservation area. A seller's "I don't think so" will not count for anything at planning.
Cirencester gives you relatively little freedom to extend or alter a home. The new-build edges are workable, but in the historic core almost every change needs consent from the council.