House Extensions & Loft Conversions across Hertfordshire

LCCL Construction is a design and build company based in Finchley N3, working across Hertfordshire 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.

Hertfordshire is a short drive up the A1 or M1 from Finchley, and the Barnet and Enfield borders (Hadley Wood, Potters Bar, Borehamwood) are where we have done some of our largest work — including a 3,033 sq ft new build. Larger plots than inner London mean wraparound and double-storey extensions are often possible, and detached 1930s and post-war houses convert well.

What it costs across Hertfordshire

The final figure depends on postcode, access and site conditions — the instant estimate applies your area. 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: £32,400 – £50,400 inc. VAT (£27,000 – £42,000 + VAT)
• Rear dormer loft: £56,400 – £80,400 inc. VAT (£47,000 – £67,000 + VAT)
• L-shaped dormer: £58,800 – £98,400 inc. VAT (£49,000 – £82,000 + VAT)
• Hip to gable: £60,000 – £114,000 inc. VAT (£50,000 – £95,000 + VAT)
• Mansard: £76,800 – £148,800 inc. VAT (£64,000 – £124,000 + VAT)
• Rear extension: £42,000 – £90,000 inc. VAT (£35,000 – £75,000 + VAT)
• Side return: £42,000 – £93,600 inc. VAT (£35,000 – £78,000 + VAT)
• Wraparound: £72,000 – £153,600 inc. VAT (£60,000 – £128,000 + VAT)
• Double storey: £90,000 – £198,000 inc. VAT (£75,000 – £165,000 + VAT)

Kitchen on top of the structure, supplied and fitted: +£9,000 – £87,360 inc. VAT (£7,500 – £72,800 + VAT) depending on size and specification. Basements: cellar refit £54,000 – £114,000 inc. VAT (£45,000 – £95,000 + VAT), lowering £114,000 – £216,000 inc. VAT (£95,000 – £180,000 + VAT), new dig £216,000 – £360,000 inc. VAT (£180,000 – £300,000 + VAT)+. New build: £2,200 – £4,500 per sqm (zero-rated for VAT).

A recent LCCL project near Hertfordshire

Lancaster Avenue, Hadley Wood EN4 — 3,033 sq ft luxury new build

A 3,033 sq ft detached house designed and built by our team from planning through to handover.

See the project

Planning across Hertfordshire

Hertsmere, St Albans and Welwyn Hatfield are generally pragmatic on permitted development and single-storey extensions; the 8m (detached) and 6m (semi) larger-home-extension route is available with prior approval. Green Belt and village conservation areas around St Albans and Hadley Wood need more care — we check the designation before design starts.

Council by council

Hertsmere (Potters Bar, Borehamwood, Elstree, Bushey, Radlett) applies its 2006 Part E extension guidelines — 3.35 m single-storey rear depth, side extensions under half the house width, rear dormers under 60% of the roof face and a 45-degree light test — with the disproportionate-additions test in the Green Belt; The Royds in Potters Bar is its one conservation area with a householder Article 4 direction. St Albans still decides under its 1994 plan while the new Local Plan completes examination, and is the only council here with a numerical Green Belt guideline (20–40% floorspace or 90–180 m³ over the original house, 2004 guidance); the city-centre conservation area and its Article 4 streets remove most permitted development. Watford adopted its Local Plan in October 2022 and keeps a detailed Residential Design Guide (3 m/3.5 m rear depths, dormers under half the roof, a codified 45-degree and 25-degree test) with ten conservation areas. Welwyn Hatfield adopted its Local Plan on 12 October 2023 and Broxbourne on 23 June 2020, the latter with new Residential Design and Amenity guidance (August 2025) that applies a 45-degree test to two-storey and a 60-degree test to ground-floor extensions. Building control across Hertsmere, Welwyn Hatfield and Broxbourne is Hertfordshire Building Control Ltd; St Albans and Watford share a service. See our St Albans, Watford, Potters Bar and Borehamwood pages for the detail.

Anderson's advice: talk to your neighbours before the notices go out

Semi-detached hip-to-gable plus rear dormer combinations almost always fall under permitted development, but neighbours often object informally. A conversation over the fence before the party wall notice lands avoids most of it.

What our clients say

“The Lccl construction team built my extension with a quick and clean service, they worked non stop through the winter weather.. I would highly recommend this company!”

— Chris Dunn, Google Reviews

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 structural warranty.

Areas we cover

Dedicated pages for the areas we work in most:

• Potters Bar
• Borehamwood
• St Albans
• Watford

We also work across Hadley Wood, Radlett, Bushey, Elstree, Shenley, Hatfield, Welwyn Garden City, Cheshunt and Harpenden — the same team, the same contract and the same price bands apply, and we survey every project in person before quoting.

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.