How Much Does a Single Storey Extension Cost in Somerset?

The realistic price range in 2026

For a straightforward single storey rear extension in the Yeovil and South Somerset area, most projects land between roughly £2,000 and £3,000 per square metre for the build itself. A modest 15 to 20 square metre extension therefore tends to fall in the £30,000 to £55,000 bracket, while a larger open-plan kitchen-diner of 30 to 40 square metres can reach £60,000 to £90,000 or more.

These figures cover the structural shell, roof, and a standard fit-out. They exclude big-ticket extras like a high-end kitchen, bifold doors, or underfloor heating, which sit on top.

• Basic to good standard finish: around £2,000 to £2,400 per m2
• High specification, glazing-heavy: £2,500 to £3,000+ per m2
• Add roughly 10 to 15 percent contingency for the unexpected

What actually drives your quote

Two extensions of identical size can differ by tens of thousands depending on the detail. Groundwork is the biggest hidden variable. Many Somerset properties sit on clay or have older drains, and if we hit poor ground or need to divert a soil pipe, foundation costs climb quickly.

The look you want matters just as much. Matching the local hamstone or reclaimed brick on a period property costs more than blockwork and render, and a lantern roof or run of bifolds can add £5,000 to £15,000 on their own.

• Ground conditions and foundation depth
• Whether you are opening up into the existing house with a steel beam
• Glazing: standard windows versus bifolds, sliders or roof lanterns
• Kitchen and bathroom specification
• Access to the rear of the property for materials and muck away

Fees and permissions in Somerset

Beyond the build cost, budget for the professional and statutory fees that catch people out. Many single storey extensions fall under permitted development, but you will still need Building Regulations approval, and a Lawful Development Certificate is worth having for peace of mind when you sell.

If you live in a conservation area, which covers parts of Yeovil, Sherborne's fringe and villages like Montacute, permitted development rights are often restricted and full planning permission may be required.

• Planning or Lawful Development Certificate: roughly £200 to £600
• Structural engineer for beam calculations: £500 to £1,200
• Building Control and inspections through Somerset Council
• Party Wall agreement if you are building near a neighbour's boundary

How to keep costs sensible

The cheapest quote is rarely the cheapest project. A fixed, itemised quote lets you compare like for like and spot where a low headline price hides thin allowances for kitchens or glazing. Ask any builder to show you the assumptions behind their number.

Simple rectangular shapes, a standard flat or mono-pitch roof, and keeping the kitchen and bathroom in sensible positions relative to existing services all trim the bill without hurting the finished space.