Single Storey Extension Cost: What Homeowners Actually Pay

Online guides quote a single number. Real projects rarely land on it exactly. Understanding how much does an extension cost for a single storey scheme means looking past the average to what actually drives the final figure.

Most homeowners start with a number in mind before design work even begins. Knowing what actually shapes that number helps set a budget that holds.

What's The Typical Price Range?

A single storey rear extension generally costs £1,500 to £2,500 per square metre. Design fees, structural work, and finishing all sit within that range for most standard projects.

This figure covers the complete project when quoted honestly, not just the visible construction work most people picture when they think about an extension.

A modest 20 to 30 square metre extension usually lands between £45,000 and £75,000 all in. Larger or more elaborate schemes push well beyond that. Location within London affects this range too. Labour and material costs vary noticeably between boroughs, even for identical project specifications.

Why Do Two Similar Extensions Cost Different Amounts?

Roof design matters more than most homeowners expect. A flat roof costs considerably less than a vaulted or lantern style structure.

Glazing choice matters too. Bi-fold or sliding doors cost more than standard windows, both in the units themselves and in the structural work needed to support larger openings.

That structural work is easy to underestimate. A wider opening often needs a stronger steel beam overhead, which adds cost beyond the glazing itself.

Site access affects cost as well. A narrow terrace with limited delivery space often costs more per square metre than a property with easy access for materials and equipment. Deliveries spread over more visits, or specialist small scale equipment, both add cost that a generic per square metre figure rarely accounts for.

What Determines Single Story Extension Cost Beyond Size?

Existing structural condition plays a real role. An older property with an already compromised wall or foundation needs remedial work a newer, better maintained property simply doesn't. A survey catches this before design work starts, which matters more than most homeowners initially realise.

Specification level shifts the final number significantly too. A basic finish sits at the lower end of any range. Bespoke joinery, underfloor heating, and premium materials push costs toward the top.

Two extensions of identical size and type can land at opposite ends of the typical range depending entirely on these choices. Checking a detailed single story extension cost breakdown helps set expectations before comparing quotes side by side.

How Much Should I Budget For Design And Planning?

Design fees typically add 8 to 15 percent to the overall build cost. This usually covers concept design, planning submission, and building regulations together.

Planning route matters for cost too. A project qualifying for Permitted Development generally moves faster and costs less in fees than one needing a full planning application. Conservation area status can add cost indirectly, since designs on protected streets often need more detailed documentation before submission.

What's Often Missing From Initial Extension Quotes?

Structural engineering sometimes gets quoted separately rather than included upfront. Party Wall Act costs, where the extension affects a shared wall, frequently appear as a later addition too.

A quote that excludes these isn't cheaper. It's incomplete. The real total surfaces eventually, usually at a less convenient point in the project.

Asking for a fully itemised quote before signing anything reveals whether these costs are already accounted for. A quote that looks cheaper upfront but excludes these items rarely ends up cheaper once the full project is complete.

How Do I Get An Accurate Estimate For My Specific Property?

A proper feasibility assessment reviews the actual property rather than applying a generic average. This covers planning status, structural condition, and realistic scope before any design fee gets spent.

That single step tends to prevent the most common and expensive mistake in extension planning, designing around a budget that was never realistic for the property in question. Homeowners who skip this step often discover the gap mid design, forcing either a smaller scope or an unplanned cost increase.

The Bottom Line

Single storey extension cost depends on far more than square footage alone. Roof design, glazing, site access, and existing structural condition all shape the final figure meaningfully.

Get a property specific estimate before comparing quotes. A generic average rarely reflects what a real project will actually cost.

Roof design, glazing, and site access all vary property by property. A budget grounded in these specifics holds up far better than one built on a national average alone.