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Two storey extensions · London & Hertfordshire
Two storey extension London: cost, planning rules and build time — from £75,000
By Anderson & the LCCL Construction Team
Finchley N3 · Design & build, LABC award winners 2024 and 2026
A two storey extension adds a bedroom, bathroom or study above the new kitchen, for far less per square metre than a single storey — the foundations, roof and scaffold are shared. It is also the extension type most likely to need a planning application. This page gives you the 2026 numbers by size, the rear, side and wraparound options, the permitted development rules that occasionally apply, and how the job runs with one team.
Quick answer
£75,000–£165,000 for a two storey extension, design and build inclusive (kitchen and bathroom fit-out on top). Postcode dependent.
Usually a planning application. Permitted development allows a two-storey rear to 3 m only if it stays 7 m from the rear boundary — most London gardens are too short.
16–24 weeks on site; 12–16 weeks of design and approvals before that.
A free 30-minute phone call with our team, no obligation.
LABC Building Excellence Award 2024
Best Residential Extension in London — Bohun Grove, East Barnet EN4
LABC Building Excellence Awards 2026
London Regional Winner, Best Conversion or Alteration — Weech Road NW6
One contract
Architecture, structural engineering, planning, construction and project management — all under one contract
Guaranteed & insured
12-month workmanship guarantee, structural guarantee to building-control sign-off, 10-year LABC Warranty available
What a two storey extension from LCCL includes
Design and build inclusive — the price covers both floors, the structure between them and the approvals.
Measured survey of the house and garden
Architectural design and drawings, with revisions until you're happy
Planning application or Lawful Development Certificate, prepared and submitted for you
Building regulations drawings and structural calculations by our chartered engineer
Party wall notices served on your behalf (£300 each, at cost)
Foundations, drainage and any build-over agreement with Thames Water
Structure on both floors, new first-floor structure, roof, windows and doors to a weather-tight shell
Plaster finish, first-fix electrics and plumbing, underfloor heating where specified
Fitting of the kitchen, flooring and second-fix items you supply
LABC inspections and completion certificate
A dedicated project manager and a weekly progress update
12-month workmanship guarantee and structural guarantee; 10-year LABC Warranty available
How much does a two storey extension cost in London in 2026?
Guide ranges, design and build inclusive. Every project is priced from your postcode and house type.
Size (both floors)
Typical use
Guide range
3 m deep, full width (≈25–30 m² total)
Kitchen-diner below, double bedroom or bathroom above
£75,000 – £100,000
4 m deep, full width (≈35–45 m²)
Open-plan kitchen below, bedroom with en-suite above
£95,000 – £130,000
Two storey side or wraparound (≈45–60 m²)
Kitchen and utility below, two rooms above — a four-bed from a three-bed
£120,000 – £165,000
Related project
Guide range
On site
Single-storey rear extension
Kitchen-diner only, usually permitted development
£35,000 – £75,000
10–14 wks
Rear extension + loft conversion
Ground floor and roof instead of a first-floor extension
See loft + extension
One programme
Side extension with room above
Two storey over a side passage on a semi
£75,000 – £120,000
16–20 wks
What the price includes: survey, architectural drawings, planning application, building-regulations drawings, structural calculations, construction of both floors to plaster finish with first-fix electrics and plumbing, LABC sign-off and guarantees. On top: kitchen and bathroom fittings (typically £15,000–£75,000 for the kitchen), flooring and second-fix items you choose. The figure on your itemised estimate is the figure you pay — nothing is added afterwards. Postcode dependent — the instant estimator adjusts the range for your area in thirty seconds, and a detailed itemised estimate follows one survey.
What moves the price: depth and width, as with any extension; the first-floor structure and the steel where the new floor meets the old house; matching brick and roof form to the existing house (planning usually requires it); windows above and glazing below; a bathroom on the first floor, which adds drainage and a soil stack; and the finish level on both floors.
Two storey extensions by house type
Semi-detached (1930s and Edwardian)
The most common two storey extension: full width across the back, 3–4 m deep, a pitched roof matching the house. A planning application is needed in nearly every case because the garden is shorter than the 7 m the permitted development rule demands, and the 45-degree rule to the attached neighbour's rear window decides the depth. Barnet, Enfield and the Hertfordshire towns approve these routinely when the roof matches.
Detached house
The best case for two storeys: more garden, no attached neighbour, and sometimes the 7 m boundary rule is met so the extension is permitted development. Side two-storey extensions must stay 2 m from the boundary at the eaves or go to planning.
Terrace
Two storey rear extensions over the outrigger are possible but the outlook and daylight tests to both neighbours are strict; many terrace clients get the same bedroom from a loft conversion instead, with no planning application. We compare both on the consultation call.
Planning
Do you need planning permission for a two storey extension?
Usually yes. Permitted development for two storeys exists, but its conditions rule out most London houses.
Route
Conditions
Time
What we do
1. Full planning
The norm — most two storey extensions fail one PD condition
8 weeks
Drawings, design statement, daylight check and application by our team
2. Permitted development
Rear only, max 3 m deep, at least 7 m from the rear boundary, roof matching the house, no balconies, side walls not within 2 m of a boundary
No application
Lawful Development Certificate where it genuinely applies
3. Pre-application advice
Conservation areas, sensitive streets, boroughs that engage
4–6 weeks
We advise whether the fee is worth it for your borough
Building regulations & party walls
The other two approvals
Every extension needs building-regulations approval whether or not it needs planning: foundations, structure, insulation, glazing area, drainage and fire. We do this through LABC with full-plans drawings and our own structural calculations, so the inspector is checking a design he has already approved, not improvising on site.
On a semi or terrace the Party Wall Act applies on both floors — excavation near the neighbour's foundations below and the new first-floor structure against the shared wall above. Notices go out at least two months before we start; most neighbours consent, and where a surveyor is appointed we manage it.
The two conditions that catch almost every London house are the 7 m distance to the rear boundary and the 2 m distance to a side boundary. If either fails, it is a planning application — which is not a reason to avoid a two storey extension, just a reason to design it for the officer from day one: matching materials, a roof that follows the house, and a depth that passes the 45-degree test to the neighbours.
Double storey extension cost guide · Planning permission for house extensions
Recent two storey extension projects
Bohun Grove, East Barnet EN4 — rear extension, hip-to-gable loft and whole-house renovation
LABC 2024 Best Residential Extension in London
Semi-detached family home One programme loft + extension + renovation
A total renovation of a family semi: a rear extension opening into a full-width kitchen and dining space with floor-to-ceiling glazing and skylights, a hip-to-gable loft conversion with a rear dormer, two Lusso Stone bathrooms, enlarged bedrooms, new windows throughout, a regulated outbuilding and landscaping.
Because the loft and extension ran under one contract, the steelwork for the open-plan ground floor and the loft floor was designed together and lifted in on the same day — one crane hire, one scaffold, one building-control file.
See the Bohun Grove project · All projects
ANDERSON · FOUNDER, LCCL CONSTRUCTION
Anderson's advice: start the party wall process the week you sign
Most clients underestimate the party wall timeline — on a semi-detached or terraced house it can add three months to the project if it is left until the drawings are finished. We serve the notices at design stage, not build stage, so the two-month notice period runs while the planning application is in.
Talk to the neighbours before the letter lands. A five-minute conversation over the fence, with the drawings in your hand, is the difference between a consent form back in a week and a surveyor's bill.
How a two storey extension runs with LCCL
From the first call to the completion certificate. Because a two storey extension almost always needs a planning application, allow 12–16 weeks for design and approvals before work starts.
WEEK 0
Free consultation call
A 30-minute phone call with our team about the house, what you want from the space and a realistic budget. Your instant estimate is the starting point.
WEEKS 1–4
Survey & design
Measured survey, architectural drawings and your detailed itemised estimate — design, planning, structure, construction and warranty priced line by line.
WEEKS 4–12
Approvals
Planning application (eight-week decision), building-regulations drawings, structural calculations, party wall notices to the neighbours. Nothing starts on site until it is signed off.
BUILD
On site
16–24 weeks on site for a two storey extension. Weekly updates from your project manager, one point of contact throughout.
HANDOVER
Sign-off
Snagging, LABC final inspection and completion certificate, guarantees in writing. 10-year LABC Warranty available.
“Everyone at LCCL Construction was skilled, trustworthy and worked to the highest standard. I am delighted with our loft conversion, it is exactly what we wanted and I highly recommend the company - they have transformed the quality of my home. Overall it…”
KashImaging · Google Reviews
“We are very pleased to have used Lccl Construction to do full house renovation and loft conversion to our edwardian terraced house. Project involved total strip out of interiors and remodelling repositioning the interior walls, electrical and plumbing…”
neil smith · Trustpilot
“We did a 3x10m single storey rear extension with LCCL recently. It was scheduled to take 12 weeks to complete but with Alex's experience and agile project management, we are ahead of schedule (finished in 10). Alex, and LCCL,is extremely knowledgeable and…”
Galichet Christophe · Google Reviews
Two storey extension questions we get asked most
£75,000 to £165,000 in 2026, design and build inclusive — survey, drawings, planning application, structural calculations, construction of both floors to plaster finish and building-control sign-off. A 3 m full-width rear sits at the bottom; a two storey side or wraparound at the top. Kitchen and bathroom fittings are on top.
Yes, usually by a clear margin. The foundations, the roof and the scaffold are shared across two floors, so the second floor costs far less than the first. That is why a two storey rear often makes more sense than a single storey plus a loft — though on a terrace the loft usually wins on planning.
Almost always. Permitted development allows a two storey rear extension of up to 3 m only if it is at least 7 m from the rear boundary, matches the roof of the house, has no balconies and keeps 2 m from any side boundary. Most London gardens fail the 7 m test, so it is a full application with an eight-week decision.
Sixteen to twenty-four weeks on site, plus 12–16 weeks for design, planning and building regulations before that. The programme is longer than a single storey because of the first-floor structure, the roof tie-in to the existing house and the bathroom services above.
Usually, but it is more disruptive than a single storey: the rear bedroom above the extension is out of use while the new floor ties in, and the roof of the house is opened at the junction. We plan the sequence with you so the disruption is concentrated into a few weeks.
If you want a bigger kitchen and a bedroom, and the garden and neighbours allow, a two storey rear does both in one build. If planning is doubtful, a single-storey rear plus a loft conversion usually goes through under permitted development and gives the same rooms. We compare the two on the consultation call.
On a semi or terrace, yes — on both floors. Notices go out at least two months before work starts; we serve them at cost (£300 each) at design stage so the period runs alongside the planning application. Most neighbours consent; where a surveyor is appointed we manage it.
For planning, usually yes on the roof form and the brick; the rear elevation can be more contemporary with large glazing below. A matching pitched roof is what gets most two storey extensions approved first time.
Where we build two storey extensions
From Finchley N3 we cover North London, North-West London, Central London, South-West and West London, and Hertfordshire — and we also work across the rest of London, Buckinghamshire, Surrey, Kent and Essex. Recent two storey extensions: Crouch End N8 and Welwyn Garden City AL7.