A kitchen extension is one of the most popular home improvement projects in the UK for good reason: it solves the most common domestic complaint, a kitchen that is too small and cut off from family life, in one hit. By pushing the back or side of the house outward, you gain the floor space to create a genuine open-plan kitchen-diner with proper dining, garden access, and natural light. This guide gives real 2026 costs, compares the main extension types, and runs through the design decisions that make or break the result.

What Does a Kitchen Extension Cost in 2026?

The cost of a kitchen extension depends on three things: the size of the extension, where you live, and what specification you choose. The table below gives all-in costs (construction + professional fees + VAT) for a single-storey kitchen extension at different sizes. Kitchen fit-out costs are shown separately because they vary enormously with the cabinet and appliance specification chosen.

Extension sizeLondon & SEMidlands / NorthNotes
8 m² (small infill)£24,000–£38,000£18,000–£27,000Side return or small rear
12 m² (typical single-storey)£34,000–£54,000£25,000–£39,000Most common size
18 m² (large open-plan)£50,000–£80,000£36,000–£58,000Full-width rear extension
25 m² (large with utility)£68,000–£110,000£49,000–£80,000Garden room feel

Kitchen fit-out (separate):

SpecificationCost, cabinets and appliances
Mid-range (e.g. IKEA or Wren with AEG appliances)£8,000–£18,000
Upper mid (e.g. Howdens Heritage or Symphony)£18,000–£35,000
High-end (bespoke or handleless with Miele/Sub-Zero)£35,000–£80,000+

Build cost per m²: £1,900–£3,200/m² in the Midlands and North; £2,400–£4,500/m² in London and the South East. Glazing-heavy designs (structural glass rooflights, bifold walls) push the per-m² cost toward and above the top of those ranges. As a cross-check, the HomeOwners Alliance puts a single-storey extension at about £1,800–£3,000 per m² across the UK and £2,300–£3,400 per m² in London.1

Open-Plan Kitchen-Diner Layouts

The point of a kitchen extension is rarely more kitchen, it is usually more life. The design that transforms a house is the open-plan kitchen-diner-living space that opens to the garden. Here are the layouts that work:

L-shaped kitchen with dining zone. The extension adds 3–4 m of depth to the rear of the existing kitchen. The original room becomes the cooking zone; the new portion is the dining and family living area. Works best when the extension is at least 4 m wide.

Full-width rear wall demolition. The entire rear wall of the ground floor is removed and spanned with a steel beam (often a 203 × 133 or 254 × 146 UB, sized by a structural engineer for the span and load). The kitchen runs left-to-right across the full 5–7 m width, with a kitchen island dividing preparation from dining. The rear wall becomes a 4–6 m bi-fold or sliding door set.

Side return infill plus short rear extension. Combining a 2 m side return infill with a 2 m rear extension produces an L-shaped extension that can add 18–25 m² to the ground floor in one project. More complex structurally (two corner steels and two sets of foundations) but the spatial result is exceptional.

Garden room annexe. A detached or semi-detached extension that is all glazing on three sides, connected to the kitchen by a short link corridor. Functions as a bright garden room and removes the visual barrier between inside and outside.

Bifolds, Sliding Doors & Rooflights

The rear wall glazing is the decision that most affects how the finished space feels.

Bi-fold doors are the popular choice for openings of 2.4–5.5 m. Panels fold back accordion-style to leave virtually no frame in the opening. Typical specification: thermally broken aluminium, double-glazed, with a whole-unit U-value of 1.4 W/m²K or better, the Part L limit for a new door with more than 60% of its face glazed.2 Cost installed: £4,500–£7,500 for a 3-panel set; £8,000–£14,000 for a 5-panel set. They need a weathertight threshold, usually with a small upstand (or specify a low-threshold version for £800–£1,500 extra).

Sliding doors (including lift-and-slide and tilt-and-slide types) suit openings of 2.0–4.0 m and give slimmer sight lines than bi-folds. A 3-panel system costs £5,500–£10,000 installed. Often better thermal performance than bi-folds in equivalent price brackets, because there are fewer frame-to-panel junctions.

Fixed glazed panels with a central door are the architectural choice, slim structural glass fins or a Schüco-type aluminium curtain wall. Budget £10,000–£22,000 for a 3.5–4.5 m run; the result is a clean, frameless look that bi-folds cannot match.

Rooflights are not optional on lean-to and flat-roof extensions. The junction between the existing house wall and the new roof creates a dark shadow if only the rear wall is glazed. One or two roof lights over the kitchen side of the extension solve this. Flush-fit rooflights (a flat-roof window or a frameless fixed rooflight) in a 600 × 900 mm or 1,000 × 2,000 mm size cost £700–£1,800 each installed. A common rule of thumb is about 1 m² of rooflight per 10 m² of floor area, within the Part L glazing allowance below.

Side Return vs Rear Extension

Which type is right for you depends on the geometry of your house and garden:

FactorSide Return ExtensionRear Extension
Best suited toVictorian/Edwardian terraces with a narrow side alleyMost house types; semi-detached, detached
Space gained8–15 m² (limited by alley width)12–35 m² depending on garden depth
Effect on gardenMinimal, uses dead spaceReduces garden length
Planning complexityPD only up to half the original house width and 4 m high; Party Wall Act commonUsually PD (single storey); Prior Approval for 3–6 m (4–8 m detached)
Cost efficiencyLower absolute cost; higher per-m²Better value for larger additions
Typical outcomeWider kitchen or kitchen-dinerLarger kitchen-diner, garden room or extra reception

Many Victorian terrace owners do both simultaneously: a combined side-and-rear (“wraparound”) extension can add 20–35 m² in one project. Under Class A, a side extension has to be single storey, no more than 4 m high and no wider than half the width of the original house.3 Where an extension fills the area between a side wall and the rear wall, the side and rear limits both apply, so a full-width wraparound usually needs planning permission.4 See the separate guide on wraparound extensions for details.

Worked Examples

Example 1, Terraced house, Hackney, East London. 3-bed Victorian terrace, 6 m wide plot. Side return 1.5 m × 5 m = 7.5 m² + short rear extension 3 m × 4 m = 12 m². Total new floor area: 19.5 m². Result: open-plan kitchen-diner with island and bi-fold rear doors. Construction cost: £68,000; kitchen fit-out (Schuller handleless): £22,000; professional fees and building control: £9,500. Total: £99,500. Value added (estate agent estimate post-completion): £115,000.

Example 2, Semi-detached house, Solihull. 4-bed 1970s semi, 8 m wide plot. Single-storey rear extension 5 m × 3.5 m = 17.5 m². Rear wall opened full width with 5-panel bi-fold. Construction cost: £38,000; mid-range kitchen fit-out: £15,000; fees: £6,500. Total: £59,500. Value added: £45,000–£55,000.

Example 3, Detached house, Nottingham. 4-bed 1990s detached. Rear extension 6 m × 4 m = 24 m². Full-width rear glazed screen with pivot door. Construction cost: £52,000; high-spec kitchen: £38,000; fees: £8,000. Total: £98,000. Additional first-floor bedroom above extension at incremental cost of £22,000 made the two-storey version the better value option.

Planning and Building Regulations

Single-storey kitchen extensions on terraced and semi-detached houses can extend up to 6 m to the rear under Permitted Development (PD) without a full planning application, provided the house is not in a conservation area or on other article 2(3) land.3 Detached houses can go to 8 m. The base Class A limit is 3 m for an attached house and 4 m for a detached one.5 Anything beyond that, up to the 6 m and 8 m figures above, requires Prior Approval under the Neighbour Consultation Scheme, a 42-day process through the council.3

Key Building Regulations requirements:

  • Part L (energy): new walls U-value ≤ 0.18 W/m²K; new roof ≤ 0.15 W/m²K; windows and glazed doors ≤ 1.4 W/m²K (whole unit); rooflights ≤ 2.2 W/m²K2
  • Part L glazing allowance: windows, rooflights and glazed doors in an extension should not normally total more than 25% of its floor area plus the area of any windows and doors it covers up; a more glazed design has to show compliance by an area-weighted U-value or SAP calculation2
  • Part A (structure): any steel beam spanning the rear wall opening needs structural calculations for building control, normally from a structural engineer6
  • Part P (electrics): new kitchen circuits must either be installed by a registered competent person who self-certifies them, or be notified to building control before work starts7
  • Part H (drainage): new drains and waste connections in the extension come under the same building control application, and building over or within 3 m of a sewer shown on the water company’s sewer records (requirement H4) must go through a full plans application rather than a building notice8

Submit a Full Plans application to building control before breaking ground. Fees: £900–£1,600 depending on project value and local authority.

Programme

Allow the following from initial design meeting to moving back into the kitchen:

StageDuration
Design, specification, Prior Approval / PD notification8–12 weeks
Building Regulations approval5–8 weeks (can overlap design)
Site works: foundations, structure, roof, glazing9–14 weeks
First fix M&E, insulation, board2–3 weeks
Kitchen installation, tiling, decoration3–5 weeks
Total elapsed time5–9 months

The kitchen will be out of commission for approximately 6–10 weeks during active construction. Plan for a temporary kitchen arrangement, a microwave, kettle, and a camping stove in the living room is the usual solution.

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.

References (8)

Figures in this note were checked against the sources below on 2 October 2026. Superscript numbers in the text point to them. Every source the site cites, by topic.

  1. 1costfairHomeOwners Alliance, guidance.RN-XKP48OA price, true at 2026-09. HomeOwners Alliance's breakdown of extension costs by type, size and location, together with the fees on top: design, structural engineer, planning, building control and party wall. A consumer body rather than a cost consultancy, so the value is in listing every cost a homeowner forgets rather than in the accuracy of any one band.
  2. 2technicalvery strongHM Government (GOV.UK).RN-IHEXOXThe full Approved Document L Volume 1 for dwellings, 2021 edition with the 2023 amendments, for use in England. This is the statutory guidance on conservation of fuel and power: the U-value tables for new and replacement elements, the rules on when an existing element must be upgraded, controls and commissioning for heating systems, and the notification requirements. If a note quotes a target U-value for a wall, roof, floor, window or door, this is where the number comes from.
  3. 3technicalvery strongThe National Archives (legislation.gov.uk), legislation.RN-Z0XQ8VParagraph A.1 sets the householder extension limits: buildings may not cover more than 50 per cent of the curtilage (A.1(b)); a single-storey rear extension may not extend more than 4 metres beyond the rear wall of a detached house or 3 metres for any other house, nor exceed 4 metres in height (A.1(f)), rising to 8 metres and 6 metres under the larger home extension route (A.1(g)); an extension of more than one storey may not extend more than 3 metres beyond the rear wall or be within 7 metres of the rear boundary (A.1(h)); eaves within 2 metres of a boundary may not exceed 3 metres (A.1(i)); a side extension may not exceed 4 metres in height, have more than one storey or be wider than half the width of the original house (A.1(j)). Paragraph A.2 removes cladding and side or multi-storey rear extensions on article 2(3) land such as conservation areas. A.3 requires materials of similar appearance and obscure-glazed, non-opening upper-floor side windows unless the opening part is more than 1.7 metres above the floor, and A.4 sets the neighbour consultation (prior approval) procedure with its 42-day determination period.
  4. 4technicalvery strongMinistry of Housing, Communities and Local Government (gov.uk), guidance.RN-3OBUOMThe government's own clause-by-clause interpretation of GPDO Schedule 2 Part 1, published 13 April 2016 and last updated 10 September 2019, and the document a planning officer will actually reach for. It explains the improvement or extension of homes without applying for planning permission where that would be out of proportion with the impact of the works, and defines the terms the Order uses that most disputes turn on: original dwellinghouse, curtilage, principal elevation, and how heights are measured on sloping ground. For Class E it works through what incidental to the enjoyment of the dwellinghouse means in practice, which is the test that excludes a garden room used as self-contained living accommodation, and how the 50 per cent curtilage calculation is done. Use it in preference to a paraphrase whenever a note has to explain why a particular garden building does or does not qualify.
  5. 5technicalvery strongThe National Archives, legislation.RN-OWGIY3Schedule 2 Part 1 of the GPDO 2015, development within the curtilage of a dwellinghouse. This is permitted development itself: Class A enlargement and alteration, with the limits and conditions, through the classes covering roof alterations, porches, outbuildings and hard surfaces. Every statement about what can be built without planning permission traces to this Part, and the exclusions at the start of each class are where most projects actually fall out.
  6. 6technicalvery strongLABC Front Door, guidance.RN-ZHMBKZLABC's homeowner guidance on steelwork in extensions. It says steel beams must be designed by a structural engineer and that the design is checked by building control.
  7. 7technicalvery strongMinistry of Housing, Communities and Local Government.RN-JYJ3I5The document that decides what electrical work a homeowner may legally do themselves, and the exact list of notifiable jobs. Regulation 12(6A), reproduced at paragraph 2.5, makes only three things notifiable: the installation of a new circuit, the replacement of a consumer unit, and any addition or alteration to existing circuits in a special location. Regulation 12(9) defines special location as, in a room containing a bath or shower, the zone extending vertically from finished floor level to 2.25 metres (or the shower head position if higher) and horizontally 600mm from the edge of the bath tub or shower tray. Paragraph 2.7 states that all other electrical installation work is not notifiable, namely additions and alterations to existing installations outside special locations, and replacements, repairs and maintenance anywhere. Paragraph 2.8 gives worked examples: a built-in cooker is not notifiable unless a new cooker circuit is needed, and connecting an electric gate or garage door to an existing isolator is not notifiable but running a new circuit from the consumer unit to that isolator is. Paragraph 2.2 confirms Part P reaches outdoor work including fixed garden lighting, pond pumps and outbuildings such as sheds, detached garages and domestic greenhouses. Notifiable work must be certified by self-certification by a registered competent person, third-party certification, or a building control body (paragraph 3.1), with the compliance certificate issued within 30 days (paragraph 3.4). Note that Approved Document P is a source of guidance rather than a ban: non-notifiable work still has to comply with BS 7671.
  8. 8technicalvery strongThe National Archives (legislation.gov.uk), legislation.RN-37ODVXRegulation 12(1) applies to anyone intending to carry out building work, replace or renovate a thermal element, change a building's energy status or make a material change of use. Regulation 12(2) requires that person to give the relevant authority either a building notice under regulation 13 or an application for building control approval with full plans under regulation 14. Full plans are compulsory where the Regulatory Reform (Fire Safety) Order 2005 applies (12(3)), where the building fronts onto a private street (12(4)) and where paragraph H4 of Schedule 1 applies (12(5)). Regulation 12(6) exempts work done by a competent person listed in Schedule 3, work listed in Schedule 4, and work inspected by a Schedule 3A third party certifier appointed before work starts. Regulation 12(6A) still requires notification for Part P electrical work consisting of a new circuit, a consumer unit replacement, or any addition or alteration to existing circuits in a special location such as the zone around a bath or shower. Regulation 12(8) allows a building notice after the event for emergency repairs. Text is up to date to 7 September 2026.

  1. House Extension Costs: Breakdown By Size & Type, HomeOwners Alliance, accessed 2 October 2026. Priced at 2026-09 and left there. hoa.org.uk

  2. Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments, HM Government (GOV.UK), accessed 7 September 2026. assets.publishing.service.gov.uk

  3. The Town and Country Planning (General Permitted Development) (England) Order 2015, Schedule 2, Part 1, Class A, The National Archives, accessed 30 September 2026. legislation.gov.uk

  4. Permitted development rights for householders: Technical Guidance, Ministry of Housing, Communities and Local Government, accessed 30 September 2026. assets.publishing.service.gov.uk

  5. The Town and Country Planning (General Permitted Development) (England) Order 2015, Schedule 2, Part 1, The National Archives, accessed 14 September 2026. legislation.gov.uk

  6. What are the building regulations issues around steelwork in my new extension?, LABC Front Door, accessed 2 October 2026. labcfrontdoor.co.uk

  7. Approved Document P: Electrical safety - Dwellings (2013 edition), HM Government, accessed 9 September 2026. assets.publishing.service.gov.uk

  8. The Building Regulations 2010, regulation 12: giving of a building notice or an application for building control approval, The National Archives, accessed 30 September 2026. legislation.gov.uk