Planning a Home Extension in Somerset: Costs, Permissions and Timelines

A well-designed extension is one of the most cost-effective ways to add both space and value to a home, often for less than the cost of moving. But a good outcome depends on getting the planning, budgeting and sequencing right from the start. Here is a practical overview for homeowners in Somerset and Devon.

Planning permission vs permitted development

Many single-storey rear extensions fall under permitted development, meaning they do not need full planning permission, provided they stay within size, height and boundary limits. Larger or two-storey extensions, properties in conservation areas, and listed buildings usually do need permission. Even under permitted development, it is wise to apply for a Lawful Development Certificate so you have proof it was allowed, which matters when you sell.

You will also need to meet Building Regulations regardless of whether planning permission is required.

What an extension typically costs

Costs vary widely with size, specification and site conditions, but the main drivers are groundworks, structural openings, the kitchen or bathroom fit-out if relevant, and the level of finish. A glazed, high-spec kitchen extension costs considerably more per square metre than a simple box room. The best approach is a clear, itemised quote so you can see exactly where the money goes.

Typical timeline

Allow time for design and drawings, then planning or a certificate (often around eight weeks), then a structural design and Building Regulations sign-off. Construction itself for a typical single-storey extension is commonly in the region of three to four months on site, weather and specification depending.

Getting it right

The projects that run smoothly are the ones planned properly before anyone breaks ground. We handle design coordination, building control and the build itself as one accountable team, see our restorations, renovations and extensions page for examples, or get in touch to discuss your project.