House Extensions & Loft Conversions in Mill Hill

LCCL Construction is a design and build company based in Finchley N3, working in Mill Hill for over 20 years. One team, one contract: architects, structural engineers, project managers and builders coordinated by LCCL take your project from first sketch to handover.

Mill Hill is where we built the Guardhouse Way kitchen extension at Millbrook Park. The area mixes new-build estates with 1930s semis on the Broadway side and larger detached houses towards Mill Hill Village — so projects range from kitchen extensions and utility rooms to wraparounds and double-storey extensions.

What it costs in Mill Hill

Outer London — base London prices. Guide ranges for 2026, design and build inclusive (architecture, structural engineering, planning or LDC, construction, project management and building control). Every project is surveyed and priced individually.

• Velux loft: £27,000–£42,000
• Rear dormer loft: £47,000–£67,000
• L-shaped dormer: £49,000–£82,000
• Hip to gable: £50,000–£95,000
• Mansard: £64,000–£124,000
• Rear extension: £35,000–£75,000
• Side return: £35,000–£78,000
• Wraparound: £60,000–£128,000
• Double storey: £75,000–£165,000

Kitchen fit-out on top of the structure: +£15,000–£75,000. Basements: cellar refit £45k–£95k, lowering £95k–£180k, new dig £180k–£300k+. New build: £2,200–£4,500 per sqm.

A recent LCCL project near Mill Hill

Guardhouse Way, Millbrook Park, Mill Hill NW7 — kitchen extension

A large modern kitchen and dining space for a family home, with a separate utility room so the kitchen stays quieter and gains cupboard space.

See the project

Planning in Mill Hill

Barnet council; Mill Hill Village is a conservation area with planning permission needed for roof and front changes. Millbrook Park homes are new-build with their own estate covenants to check before extending.

Conservation areas and Article 4 directions in Mill Hill

The Mill Hill conservation area (Article 4 directions 1970 and 2007) covers about 260 houses along The Ridgeway and around Milespit Hill, Wills Grove and Winterstoke Gardens — brick and weatherboard cottages typical of rural Middlesex. Inside it, replacement windows and doors, roof extensions and rooflights, changes to wall materials or painting and front porches all need planning permission. Mill Hill East, Mill Hill Broadway and the interwar streets off Hale Lane and Watford Way are outside the conservation area, and there most lofts and rear extensions proceed under permitted development with a Lawful Development Certificate. Millbrook Park homes are new build with estate covenants that need checking before any extension. The Green Belt around Mill Hill limits extensions to roughly 25% of the original volume. Since May 2016 a borough-wide Article 4 direction has required planning permission to convert a house into a small HMO.

Extension size rules Barnet applies (SPD adopted 15 June 2026)

Barnet adopted its Sustainable Design and Development Guidance SPD on 15 June 2026, replacing the 2016 Residential Design Guidance, and Part 4 of it sets the numbers officers use alongside Local Plan policy CDH05. Single-storey rear extensions can run the full width of the house to a maximum depth of 3 m on a terrace, 3.5 m on a semi and 4 m on a detached house (paras 4.37–4.39), no more than half the garden length (4.40), up to 4 m high with a pitched roof but with eaves no higher than 3 m (4.41–4.42). A larger home extension approved under prior approval (6 m on a semi or terrace, 8 m detached) is accepted as a fallback position (4.45). Two-storey rear extensions are limited to half the width of the house and 3 m deep, kept 2 m from an attached neighbour’s boundary or 1 m from a detached one, with a pitched roof matching the original (4.53–4.57). Side extensions should be no more than half the original width; at first floor they need a 1 m gap to the boundary, a 0.5 m set-back from the front wall and a ridge 0.5 m below the main roof (4.69–4.74). Rear dormers should sit at least 0.5 m in from the eaves, ridge and edges of the roof (4.87); L-shaped dormers over an outrigger are limited to half its depth (4.95); front dormers are generally not supported (4.85), while a hip-to-gable that re-balances a pair of semis is (4.84). Basements are single storey with 2.5 m head height, project no further than the rear-extension limits above, take up no more than half the garden length and have front lightwells of no more than 1 m (4.103–4.121). In the Green Belt the original house should not grow by more than 25% by volume (4.163).

Light to neighbours

The SPD does not set a numerical daylight test; it asks officers to weigh loss of light to main windows of neighbours’ habitable rooms (para 4.7) and to reduce depth or height, or set off the boundary, where levels or building lines differ (4.8). In practice we design every Barnet extension to clear a 45-degree line taken from the centre of the nearest neighbouring habitable-room window, on plan and on elevation — the rule of thumb most London officers use — so the scheme does not need a separate daylight and sunlight report.

Fees, timescales and building control

A householder planning application costs £548 from April 2026 (Planning Portal service charge on top); a Lawful Development Certificate for a proposed use is half that. Barnet has eight weeks from validation to decide a householder application; larger-home-extension prior approval is a 42-day route with a £249 fee. Party wall notices run alongside: two months for works to a shared wall, one month for excavations within 3 m or 6 m of a neighbour’s foundations. We use LABC Barnet on every project, so the inspector is the council’s own. Typical inspection points are commencement, foundation trenches, oversite and damp-proof course, drainage before backfill, structural steels and joists before they are covered, insulation and fire-stopping before plastering, then completion. We book each one and send you the completion certificate at handover.

Anderson's advice: put the rooflights over the island

Rooflights over kitchen islands transform natural light more than any other single design decision. We position them on the plan before the kitchen layout is fixed, not after.

What our clients say

“LCCL Construction were professional from start to finish. The team were trustworthy, reliable and delivered work to the highest standard.”

— KashImaging, Trustpilot

As LABC Building Excellence Award winners in 2024 (Best Residential Extension in London) and 2026 (Best Conversion or Alteration to an Existing Home, Weech Road NW6), every project is delivered by one team of architects, structural engineers, project managers and builders. Workmanship is guaranteed for 12 months from practical completion, with a 10-year LABC Warranty available to purchase.

Nearby areas

Mill Hill is part of our North West London guide. Nearby areas: Hampstead, St John's Wood, Queen's Park.

To book a free consultation call LCCL Construction on 020 8037 7332 or get an instant estimate online. Get an instant estimate or book a free consultation.

One team, one contract, one point of contact — from first sketch to handover.