You can extend in Henley, but expect to ask permission more often than you'd like. Nearly four hundred buildings are listed. Four conservation areas cover much of the town. The neighbours file planning applications at a steady clip. The rear extension is a negotiation, not a weekend project.
Historic England's register lists 386 buildings in the area. Three are grade I, 19 are grade II* and 364 are grade II. Listing covers the whole building, inside and out, not just the part facing the street, so almost any change that affects its character needs listed building consent on top of any planning permission. Grade II is the most common grade and the least strict, but it still covers the windows, the internal walls and the fireplace you were planning to open up. Doing the work without consent is a criminal offence, not an administrative slip, and a later buyer's solicitor will ask about it.
Four conservation areas sit within the area: Henley Main, Reading Road, St Mark's Road and Fawley Court. Inside one, the council takes a closer interest in anything visible from the street, including extensions, roof changes, demolition and work to trees, and some changes that would be automatic elsewhere need an application here. Where a house stands in a conservation area but isn't listed, the rules are lighter, but the bar is still set by what the street looks like now.
There is one scheduled monument in the area, the strictest level of heritage protection. It matters mainly if your land sits next to it, and for most buyers it won't come up at all.
None of this has stopped people trying. South Oxfordshire's planning records show 1,927 applications in the area between April 2024 and June 2026, with 852 in the last twelve months alone. That's the town's way of saying it wants a bigger kitchen and knows it'll have to ask. A high volume shows what people are applying for. It doesn't tell you what's being approved, and nothing here should be read as a guide to your chances.
The ground adds one more item to the list. The British Geological Survey rates shrink-swell risk at class 3 across the whole area, 100% of it. Shrink-swell describes clay soils that swell when wet and shrink when dry, which can move foundations over time. It's a published hazard class, not a warning to stay away, but it does mean an extension should come with a structural engineer's view on foundation depth before the builder quotes. Nearby trees, which clay soils and thirsty roots make into a double problem, deserve a look too.
In practice, a house outside the conservation areas and off the listed register may be able to use permitted development rights for smaller extensions without a full application, subject to the national limits. A listed house, or one inside a conservation area, should start from the assumption that consent will be needed and budget the time for it. Check the listing and conservation status of the exact address before offering, not after the architect's first invoice.
Freedom to alter a home in Henley is poor: heritage protection is dense, the conservation areas cover the town's oldest streets, and the clay has its own opinions. Buy the house with the space you need already built, rather than the one you intend to extend.