House Extension: The Complete Guide

A house extension remains the default way UK homeowners buy themselves more space without moving. In 2026, a modest single-storey rear extension costs £20,000–£45,000, a full-width kitchen-diner extension runs £45,000–£90,000, and a double-storey addition that adds a bedroom upstairs typically costs £55,000–£160,000 or more. The right choice depends less on budget alone than on your plot, your roof, and whether you need one extra room or two.

This guide pulls together every extension article on the site into one place: how to choose between single and double storey, what you’ll really pay per square metre, the wraparound and side-return options specific to Victorian terraces, the planning and permitted development rules that decide whether you need an application at all, the design decisions that separate a good extension from a great one, and what actually happens once you break ground.¹

Single Storey or Double Storey?

The first fork in the road is how many storeys to build. A single storey extension is cheaper, faster, and covered by more generous permitted development rules, while a double storey extension shares foundations and roof across two floors, making the incremental cost of a first-floor room surprisingly good value.¹

If the room you actually need is a bedroom, a bedroom extension cost breakdown compares the single- and two-storey routes to adding one. And if you’re not sure extending is even the right call, should I extend or move runs the true transaction cost of moving against the cost of staying and building.

Cost Per m² and by Type

Figures assume a standard project outside central London; add roughly 15–30% for London and the South East.

Per-square-metre pricing is the fastest way to sanity-check a builder’s quote. The main house extension cost guide covers every type and specification level for 2026, while extension cost per square metre and extension cost per m2 both break down what a quoted rate should and shouldn’t include, worth reading both since contractors rarely quote like-for-like.

Match your drawings, not your description

Get three quotes from the same set of architectural drawings, not from a verbal brief. Extension quotes for the same project often vary widely between contractors, and much of that gap disappears once every quote prices exactly the same specification.

On returns, does an extension add value sets out the UK evidence on uplift versus your street’s ceiling price, and loft conversion vs extension compares the two most common ways to add space if you’re weighing up a roof project instead.

Nationwide’s analysis finds that a 10% increase in floor space, other things equal, adds about 5% to the price of a typical house.² Its 2025 research puts the gain from an extension or loft conversion that adds a bedroom at up to 24%.³

Wraparound, Side-Return and Other Layouts

Victorian and Edwardian terraces have a specific extension geometry worth understanding on its own terms. A side return extension fills the narrow alley beside the kitchen for roughly £25,000–£85,000, and a rear extension pushes straight out from the back wall, combine both in a single project and you get a wraparound extension cost of £38,000–£145,000 for the most dramatic ground-floor transformation available on a terrace.

If the real goal is simply a bigger kitchen rather than a full wraparound, kitchen extension cost covers open-plan layouts, bifolds and rooflights with worked examples. And if you’d rather keep the new space detached from the house entirely, garden room versus extension compares the two on cost, planning and everyday practicality.

Planning Permission and Permitted Development

Many single-storey rear extensions fall under permitted development, but the size limits are exact and unforgiving: extension planning permission covers the PD depth limits, the Larger Home Extension Scheme, and when you need a full application. Under Class A, a single-storey rear extension may project up to 3 m beyond the original rear wall, or 4 m for a detached house, and, outside conservation areas and other protected land, up to 6 m or 8 m through the prior approval route.⁴ Planning consent and Building Regulations are entirely separate hurdles. Permitted development only removes the need for a planning application: most extensions still need Building Regulations approval.⁵ Extension building regulations is the complete rundown of what Building Control actually checks, from foundations to insulation.

Before you finalise a design that runs close to a boundary, read right to light. A neighbour’s private right to light exists independently of your planning permission, and it’s far cheaper to design around it now than defend a claim later.⁶

Design Decisions

Good extensions are won or lost on light, not floor area. House extension design covers rooflights, matching old brick to new, and flooring continuity between the existing house and the addition, while single storey extension ideas works through roof types, flat, pitched or lantern, and the layouts that actually earn their keep.

If you want the rear wall to disappear entirely, a glass extension using structural glazing or a Crittall-style steel frame is the boldest option, though summer overheating needs designing out from day one. For a lighter-touch middle ground between a conservatory and a full extension, orangery compares all three on cost, thermal performance and resale value. Conservatories, orangeries, sunrooms, garden rooms and roof lanterns are compared side by side in the glazed extensions guide.

The Build

Conventional blockwork isn’t the only way to build. An oak frame extension cost runs well above masonry (£2,500–£4,500 per m² in the table above, against £1,800–£3,800 for a standard single-storey build) but delivers an exposed structural aesthetic no other method matches, while a SIPs extension cost lands in a similar bracket overall, the real saving is speed, reaching a weathertight shell in days rather than weeks.

Not every extension is about the kitchen: a garage extension cost guide covers attached and detached options, from a basic single-car structure to a full room above, if what you actually need is storage, a workshop, or space for a car. At the small end of the scale, adding a porch walks through types, Building Regs and quotes, and porches and canopies compares an enclosed porch with a simple canopy on cost and planning.

Where to Start

Start with the main house extension cost guide to set a realistic budget, then check your extension planning permission position before you commission any drawings, it shapes everything from depth to roof height. If your garden is narrow and your kitchen is the real problem, side return extension is usually the highest-value place to look first.

The rules in this note are those that apply in England. Scotland, Wales and Northern Ireland have their own building standards and permitted development rights.