Few London postcodes carry the architectural weight and residential prestige of Kensington. Spanning W8 and parts of W14 within the Royal Borough of Kensington and Chelsea, this is a neighbourhood defined by grand stucco-fronted Victorian townhouses, elegant garden squares, and some of the most coveted residential streets in the country. From the leafy roads surrounding Holland Park and the handsome terraces of Edwardes Square, to the quieter family streets off Kensington High Street and the distinguished properties lining Abingdon Road and Stanford Road, Kensington combines architectural beauty with genuine liveability.
For homeowners in W8 and W14, the case for extending rather than relocating is particularly compelling. With property values among the highest in the capital and a planning environment that rewards thoughtful, high-quality design, a well-conceived extension in Kensington delivers lasting improvements to how a home functions — and to its long-term worth. This guide is written specifically for Kensington homeowners and reflects Buildaway's experience delivering compliant, certified extensions across the borough, underpinned by transparent pricing and an 18-month labour warranty on all works.
How Much Does a House Extension Cost in Kensington (2026)?
Kensington sits firmly at the premium end of London's construction market. The combination of high labour costs, exacting finish standards, heritage considerations, and the logistical demands of building in a dense inner-London setting all contribute to costs that reflect the calibre of the area. Below are realistic 2026 mid-market figures:
Single-Storey Rear Extension
- Build-only: £3,300 – £4,400 per m²
- Fully finished with kitchen: £4,400 – £5,800 per m²
- Example: 25 m² kitchen-diner = £82,500 – £110,000 build-only or £110,000 – £145,000 turnkey.
Double-Storey Rear Extension
- Build-only: £3,100 – £4,100 per m²
- Fully finished: £4,100 – £5,400 per m²
- Example: 40 m² across two floors = £124,000 – £164,000 build-only or £164,000 – £216,000 turnkey.
Side Return & Wrap-Around Extensions
Highly sought after among Kensington's Victorian terrace owners: £3,300 – £4,600 per m², depending on heritage requirements, structural complexity, and glazing specification.
These figures represent compliant, Building Control certified builds appropriate for Kensington's demanding standards and RBKC's planning framework.
What Drives the Price Up or Down
- Overall floor area — the single largest determinant of cost; every additional square metre adds proportionally to both structural and finishing expenditure.
- Load-bearing alterations and steelwork — Victorian and Edwardian properties throughout Kensington almost invariably involve structural walls; RSJ installation typically adds £3,500–£7,500 to overall costs.
- Listed building status — a number of Kensington properties carry listed building designation, which imposes additional consent requirements and restricts both materials and methods of construction, adding to both cost and timeline.
- Conservation area obligations — Holland Park, Edwardes Square, and Pemberton Gardens all fall within RBKC conservation areas, requiring materials and design approaches that satisfy heritage officers before consent is granted.
- Glazing and door specification — slim steel-framed glazing, bespoke casements, and high-performance sliding or bi-fold systems suited to Kensington's architecture add £5,000–£13,000+ depending on scale.
- Basement and lower-ground extensions — increasingly popular in Kensington where garden space is finite; structurally complex and subject to specific RBKC policy, but capable of delivering significant additional usable area.
- Kitchen and bathroom fit-out — premium kitchens in this market commonly run from £18,000 to £45,000+; high-specification bathrooms from £9,000 to £22,000.
- Heating and plant — Megaflo unvented systems or full boiler replacements: £3,500–£6,000.
- Access and logistics — permit parking zones, restricted delivery windows, and the proximity of other residents in Kensington's townhouse streets all factor into site preliminaries and programme planning.
Planning Permission & Building Control in Kensington
- Permitted Development (PD): Modest single-storey rear extensions may qualify under PD rights, provided they satisfy RBKC's specific criteria for height, volume, and materials. W8 properties warrant careful assessment before assuming PD applies.
- Full Planning Permission: Necessary for double-storey extensions, any alteration visible from the public highway, wrap-around designs, and all proposals that fall outside PD thresholds or that affect a listed building or its curtilage.
- Listed Building Consent: Properties on RBKC's statutory list — and there are many in Kensington — require a separate listed building consent application in addition to any planning permission. Works must preserve the character and fabric of the listed structure.
- Conservation Areas: Holland Park Conservation Area, Edwardes Square Conservation Area, and several others covering Kensington's residential streets impose strict design controls. RBKC heritage officers assess proposals for material compatibility, visual impact, and architectural appropriateness before granting consent.
- Party Wall Act: Given the density of Kensington's terrace and townhouse stock, the Party Wall etc. Act 1996 applies to the overwhelming majority of extension projects. Formal notices must be served on adjoining owners prior to any excavation or structural work.
- Building Control: Required for all extension work — encompassing structural calculations, insulation and thermal compliance, drainage modifications, fire separation, and electrical installation sign-off. Buildaway works with RBKC's building control team and approved private inspectors alike.
Buildaway has an established working relationship with RBKC's planning and heritage departments and routinely engages pre-application advice services to reduce the risk of refusal on complex or sensitive proposals.
How Long Will It Take?
- Design and planning preparation: 5–12 weeks
- Technical drawings, listed building/conservation applications and statutory notices: 4–10 weeks (listed building applications can extend this further)
- On-site construction:
- Single-storey: 10–16 weeks
- Double-storey: 16–24 weeks
- Bespoke glazing, heritage joinery, premium kitchen installations, or basement works may extend the programme considerably beyond these ranges.
Typical Full Project Budget Breakdown (30 m² Double-Storey)
| Element | Typical Cost |
|---|---|
| Core structure & shell | £92,000 – £115,000 |
| Windows / doors / glazing | £8,000 – £15,000 |
| Kitchen supply & fit | £18,000 – £40,000 |
| Bathroom fit-out | £9,000 – £20,000 |
| Heating / boiler / Megaflo | £3,500 – £6,000 |
| Flooring & decorating | £9,000 – £22,000 |
| Professional fees & permissions | £6,000 – £14,000 |
| Total realistic range: | £145,000 – £210,000 |
(Properties with listed building status, basement components, or premium bespoke specifications can exceed £275,000+.)
Why Kensington Clients Choose Buildaway
- Deep RBKC knowledge — practical, hands-on experience navigating Kensington's conservation areas, listed building requirements, and the particular planning sensitivities of W8 and W14.
- Full transparency on costs — detailed, itemised quotations with staged payment schedules and no mid-project surprises.
- 18-month labour warranty — a longer post-completion guarantee than most comparable contractors in the premium London market.
- Accredited and fully insured — Gas Safe registered, NICEIC certified, and comprehensively covered with public liability insurance on every project.
- Integrated project delivery — Buildaway coordinates every professional discipline, from architects and heritage consultants to structural engineers and specialist trades, managing the full process so Kensington homeowners don't have to.