Single Storey Rear Extension: Design, Cost and Planning Guide for UK Homeowners

A single storey rear extension is consistently one of the most popular home improvements in the UK, and for good reason: it adds usable floor space, creates the potential for a kitchen-diner that transforms family living, and — in the right location — adds 5–15% to property value. This guide covers the planning rules, design decisions, current build costs, and the full project timeline from first sketch to handover.

Permitted Development: The Rules Explained

For most houses (not flats, listed buildings, or properties in designated areas), a single storey rear extension can be built under permitted development (PD) rights — without the need for a full planning application.

Under Class A of Part 1 of the Town and Country Planning (General Permitted Development) (England) Order 2015 (as amended), the conditions are:

Maximum depth: 3 metres for a semi-detached or terraced house; 4 metres for a detached house
Maximum eaves height: must not exceed the existing eaves height of the original dwelling
Maximum overall height: must not exceed 4 metres for a dual-pitched roof or 3 metres for any other roof type
Materials: must be of similar appearance to the existing dwelling
Side elevation glazing: no unobscured glazing on side-facing walls or roof slopes where they face a highway or boundary

These are baseline conditions. You lose PD rights entirely if:

The property is in an Article 4 Direction area (removing PD rights, typically in conservation areas or certain residential estates)
The building is listed
The property is a flat or maisonette
Previous extensions have already consumed the PD allowance (the permitted development limits apply to the original dwelling, not incrementally)

Always check with the local planning authority (LPA) before assuming PD rights apply. A Lawful Development Certificate (LDC) at £258 provides formal written confirmation that is invaluable for future sale or remortgage.

The Neighbour Consultation Scheme (Prior Approval)

If you want to build deeper than the standard PD limits — up to 6 metres for a semi-detached or terraced house or 8 metres for a detached house — you can apply through the Householder Prior Approval process (Class A.1(f)), also known as the Neighbour Consultation Scheme.

Under this process:

You notify the LPA and pay a fee of £120
The LPA notifies adjoining owners and gives them 21 days to raise objections
If objections are received, the LPA assesses the impact on the amenity of adjoining properties only
If no objections are raised, Prior Approval is automatically granted after 42 days

Prior Approval is not a full planning permission — it is a narrower check focused solely on the impact on neighbours. It does not assess design quality, materials, or streetscape character.

When You Need Full Planning Permission

You will need a full planning application (fee: £258 in England) if:

You want to build deeper than 6m/8m
The property is in a conservation area, Area of Outstanding Natural Beauty (AONB), National Park, or World Heritage Site
The building is listed (listed building consent is also required)
Previous extensions have consumed the PD allowance
The LPA has an Article 4 Direction removing PD rights in your area

Full applications target an 8-week determination period, though complex or contested applications regularly take longer.

Design Considerations

Glazing: Bifolds vs Slide-and-Turn vs Fixed

The rear wall of a single storey extension is typically given over to glazing to maximise light and connection to the garden. The main options:

Glazing Type	Typical Opening Width	Installed Cost	Pros	Cons
Bifold doors	Up to 7m+	£3,500–£12,000	Maximum opening; popular aesthetic	Many moving parts; more maintenance
Slide-and-turn (e.g. Schüco)	Up to 6m	£4,000–£14,000	Clean sightlines when fully open	Higher cost per m²
Fixed glazing + separate door	Any width	£2,000–£8,000	Thermally efficient; simple	No full opening

For roof glazing, a roof lantern or structural glass rooflight dramatically increases natural light in the room. A good-quality aluminium roof lantern (2m × 1m) typically costs £1,500–£3,500 supplied and fitted.

Internal Wall Removal and RSJ Calculations

Most single storey rear extensions involve removing a section of the existing rear wall, which may be load-bearing. A structural engineer must calculate the correct steel beam (RSJ or universal column) size and bearing length before Building Control approval can be issued. This is a legal requirement under Part A (Structure) of the Building Regulations.

A typical structural engineer's calculation package for a single beam costs £300–£600. Budget for this separately from your main contractor quote.

Party Wall Act Considerations

The Party Wall etc. Act 1996 may apply even for a standard rear extension:

Excavations within 3 metres of a neighbouring building (measured to the nearest part of the structure, not the boundary) to a depth greater than the neighbour's foundations require a Section 6 notice — serve at least 1 month before works
Work to a shared wall — for example, cutting into a party wall to insert an RSJ or remove a chimney breast — requires a Section 2 Party Structure Notice — serve at least 2 months before works

If your extension is set away from both side boundaries and involves no work to a shared wall, the Act may not apply. If in doubt, consult a party wall surveyor — most will advise on whether notice is required for a fixed fee of £100–£200.

Build Costs 2025

Specification	Cost per m²	Example: 20m² Extension
Basic (standard finishes, straightforward structure)	£1,800–£2,200/m²	£36,000–£44,000
Mid-spec (underfloor heating, quality glazing, good kitchen)	£2,200–£2,800/m²	£44,000–£56,000
High-spec (premium glazing, stone flooring, structural steelwork)	£2,800–£3,500/m²	£56,000–£70,000

These figures cover the construction of the extension shell including foundations, walls, roof structure, glazing, first and second fix trades, plastering, and decoration. They exclude:

Planning or Prior Approval fees (£120–£258)
Structural engineer's calculations (£300–£600)
Building Regulations application (£206–£1,500+ depending on LPA)
Party wall surveyor fees (£700–£4,000+ if applicable)
Kitchen or bathroom fit-out beyond standard
VAT at 20% on all works

Always include a 10–15% contingency for unforeseen groundworks, structural issues, and variations.

Adding Value: What to Expect

A well-designed single storey rear extension typically adds 5–15% to property value, depending on location, quality of execution, and how the existing layout is improved. The return is highest where:

The existing ground floor layout is genuinely transformed — for example, moving the kitchen into the extension and opening the original kitchen space into a through-room
The extension is light-filled and integrates the interior with the garden
The property is in a high-demand area where additional square metres command a meaningful price premium

In lower-value or rural areas, build cost may approach or exceed the value added. Commission a pre-project appraisal from a local RICS-registered valuer before committing to a specification.

Typical Project Timeline

Stage	Duration
Design and initial specification	3–6 weeks
Planning or Prior Approval	6–10 weeks
Party wall notice period (if applicable, can run concurrently)	4–8 weeks
Tender and contractor appointment	2–4 weeks
Build programme on site	8–14 weeks
Snagging and final Building Control sign-off	1–2 weeks
Total: design to handover	3–6 months

A single storey rear extension is one of the most straightforward and value-accretive home improvements available to UK homeowners — but get the planning rules right before commissioning drawings, appoint a structural engineer before any walls come down, and build in contingency for what's underground.