Rear Extension Cost UK 2025: Full Price Guide by Size, Finish and Location

A rear extension is the most popular type of house extension in the UK — adding space to the back of the house to create a larger kitchen-diner, family room, or study. In 2025, the cost of a rear extension ranges from £25,000 for a small basic addition to £90,000+ for a premium open-plan kitchen-diner with structural glazing and high-end finishes. This guide breaks down every cost factor so you can build a realistic budget.

Rear Extension Cost at a Glance (2025)

Size	Budget Finish	Standard Finish	Premium Finish
3m × 3m (9 sqm)	£20,700	£29,700	£40,500
3m × 4m (12 sqm)	£27,600	£39,600	£54,000
4m × 4m (16 sqm)	£36,800	£52,800	£72,000
4m × 5m (20 sqm)	£46,000	£66,000	£90,000
5m × 5m (25 sqm)	£57,500	£82,500	£112,500

Rates used: budget £2,300/sqm, standard £3,300/sqm, premium £4,500/sqm. Includes structural work, build, first and second fix, and finishes. VAT (20%) additional. London adds 20–30%.

What Does Each Finish Level Include?

Budget Finish (£2,000–£2,500/sqm)

• Standard brick-and-block cavity wall construction
• GRP or EPDM flat roof
• PVCu French or single sliding patio doors
• 1–2 PVCu windows
• Standard floor insulation and screed
• Plasterboard and skim throughout
• Builder’s range flooring (laminate or basic tile)
• Basic electrical first and second fix

Standard Finish (£2,800–£3,800/sqm)

• Brick-and-block cavity wall with feature external brick or render
• Quality flat roof with rooflight (600×900mm or larger)
• Aluminium bi-fold or aluminium sliding doors (2.4–3.6m)
• Double-glazed aluminium windows
• Underfloor heating (electric or wet system)
• Porcelain or engineered timber flooring
• USB/data sockets, pendant lighting pre-wired
• Quality plasterwork and smooth finish

Premium Finish (£4,000–£5,500/sqm)

• Structural glass roof or large glass lantern
• Full-width bi-fold or slim-frame sliding doors (4.8m+)
• Polished concrete or large-format stone flooring
• Wet underfloor heating throughout
• Feature brickwork or Corten steel cladding
• Designed lighting scheme with dimmer control
• Premium plasterwork with curved or shadow-gap details
• Open-plan kitchen integration budgeted separately

Key Cost Drivers for Rear Extensions

Structural Steel

Most rear extensions require at least one steel beam to span the opening where the rear wall of the original house is removed to connect to the new extension. Each beam (supplied, delivered, and installed with padstones and fire protection) typically costs £3,000–£7,000 depending on span and weight. A wider opening (5m+) or removal of a load-bearing chimney breast adds cost.

Glazing and Doors

The single biggest variable in rear extension costs is glazing. Costs range from:

• PVCu French doors: £800–£1,800
• Aluminium bi-fold doors (3m wide): £3,000–£5,500
• Aluminium bi-fold doors (4.8m wide): £5,500–£9,000
• Roof lantern (1.5m × 0.9m): £2,000–£4,500
• Structural glass roof: £8,000–£20,000

Foundation Type

Standard strip foundations on stable ground: £4,000–£8,000 for a typical rear extension. If the ground investigation reveals poor bearing capacity, clay shrinkage problems, or proximity to trees, you may need pile foundations (£8,000–£18,000) or a raft foundation (£6,000–£12,000).

Location Premium

Build costs in London run 25–35% above the national average. The South East (excluding London) runs 10–20% above. Northern England and Wales are typically at or below the national average. Labour is the primary driver of regional cost differences.

Additional Costs to Budget Separately

Item	Typical Cost
Architect fees (design, planning, building regs)	£4,500–£10,000
Structural engineer	£900–£2,500
Planning application fee (householder)	£258
Building regulations application	£700–£1,400
Party wall surveyor (if applicable)	£800–£2,500
Kitchen (fitted — see below)	£8,000–£40,000+
Landscaping / new patio	£2,000–£10,000
Contingency (allow 10–15%)	Varies

Rear Extension vs Other Extension Types: Cost Comparison

Extension Type	Typical Total Cost (Standard Finish)	Floor Area Added
Single-storey rear (4m × 4m)	£50,000–£65,000	16 sqm
Single-storey side return (2.5m × 5m)	£45,000–£65,000	12.5 sqm
Wraparound (side + rear)	£75,000–£110,000	25–35 sqm
Double storey rear (4m × 4m)	£80,000–£110,000	32 sqm
Loft conversion (rear dormer)	£45,000–£70,000	25–35 sqm

Does a Rear Extension Need Planning Permission?

Many single-storey rear extensions are permitted development. The limits are:

• Up to 3m depth (semi-detached/terraced) or 4m depth (detached): no planning permission needed
• 3–6m (semi/terraced) or 4–8m (detached): Prior Approval required (42-day neighbour consultation)
• Anything larger, or any property in a conservation area or with Article 4 Direction: full planning application required

FAQs: Rear Extension Costs

What is the cheapest type of rear extension?

A small brick-and-block rear extension with a flat GRP roof, PVCu doors, and basic finishes is the cheapest type. A 3m × 3m extension done to budget spec can be achieved for £18,000–£25,000 in the Midlands and North, or £25,000–£35,000 in London.

Should I include the kitchen in my extension budget?

The kitchen (units, appliances, worktops) is a separate budget line. Fitted kitchens range from £8,000 (IKEA/Howdens with appliances) to £40,000+ for bespoke. Build this into your overall project budget from the outset — many homeowners underestimate this cost.

How long does a rear extension take to build?

Typically 10–16 weeks on site for a standard single-storey rear extension. Including design, planning, and building regulations, allow 9–14 months total from first meeting to moving in. See our house extension timeline guide.

Can I get an accurate quote before my architect has done drawings?

Only rough ballpark figures — like those on this page. Accurate quotes require planning drawings that define the exact size, specification, and structural approach. Getting a builder to quote from a vague brief results in incomparable quotes that frequently result in disputes during construction.

Does a rear extension add value to my home?

A well-designed rear extension typically adds 8–15% to property value. In London and the South East, a quality open-plan kitchen-diner extension often adds more value than it costs to build — particularly on mid-terrace Victorian and Edwardian houses where the lack of kitchen space is a significant constraint.

Get a Rear Extension Design and Cost Estimate

Crown Architecture provides full design and project management for rear extensions across London and the UK. Use the form above or call 07443804841 to discuss your project and get a realistic cost estimate based on your specific property.