House Extensions Borehamwood WD6 2026: Costs, Hertsmere Planning and Two Real LCCL Projects

By Anderson & the LCCL Construction Team | Finchley N3

Borehamwood and Elstree sit just over the Hertfordshire border from our base in Finchley N3 — twenty minutes up the A1 — and WD6 is one of the areas we have built in most. The housing is mainly 1930s to 1960s semi-detached and detached with decent plots, which makes it good extension territory: there is usually room for a full-width rear extension, often a side extension, and the gardens take a double-storey addition without overshadowing the neighbours.

This guide gives our 2026 ranges for each extension type in WD6, explains how Hertsmere Borough Council treats them, and uses two projects we have completed in Borehamwood — a 4 m rear kitchen extension and a hip-to-gable loft conversion in Elstree — to show what the work looks like.

Our rear kitchen extension in Borehamwood WD6 at completion: full-width glazing onto the garden, with the old kitchen behind it turned into a utility room.

House extension costs in Borehamwood WD6 (2026)

WD6 sits in our Hertfordshire band, slightly below the London base, so expect figures in the lower half of these ranges; the instant estimate sets the band for your postcode. All inc. VAT with the ex-VAT figure in brackets, for a shell-and-plaster finish; kitchens, structural openings into the existing house, decorated and fully finished levels and fees are separate lines.

• Single-storey rear extension (15–25 m²): £42,000 – £90,000 (£35,000 – £75,000 + VAT). A typical 20 m² rear extension in WD6 is around £43,000 – £59,000 inc. VAT.

• Side extension on a semi or detached (12–20 m²): £42,000 – £93,600 (£35,000 – £78,000 + VAT).

• Side + rear or wraparound (25–40 m²): £72,000 – £153,600 (£60,000 – £128,000 + VAT).

• Double-storey rear extension (30–50 m²): £90,000 – £198,000 (£75,000 – £165,000 + VAT) — the way to add a bedroom and bathroom above the new kitchen.

• Double-storey side extension (24–44 m²): £90,000 – £198,000 (£75,000 – £165,000 + VAT), common on the detached houses with a garage to one side.

A kitchen supplied and fitted adds from £9,000 inc. VAT (£7,500 + VAT) for a small economy kitchen; a medium kitchen at mid-range specification is £22,000 – £31,000 inc. VAT. Our rear extension cost guide explains what moves a figure within these ranges, and the St Albans and Hertford guides cover the neighbouring Hertfordshire towns.

Planning in Borehamwood: Hertsmere and permitted development

Borehamwood and Elstree are in Hertsmere Borough Council's area. On an ordinary house outside a conservation area the national permitted development rules apply: a single-storey rear extension of up to 3 m on a semi or 4 m on a detached house as of right, or 6 m and 8 m under the prior approval (larger home extension) scheme; a single-storey side extension of up to half the width of the original house; and no double-storey extension without a householder application. Elstree village has a conservation area where those rights are cut back and the council expects materials to match.

Two things come up repeatedly in WD6. First, a lot of the 1950s and 1960s houses have already had a side garage conversion or a small rear addition, and the permitted development allowance is measured against the original house, so what is left may be less than you think. Second, on the newer developments around the town the original planning permission often removed permitted development rights altogether, which means a householder application for any extension — the decision notice for the estate tells you. We check both before drawing anything. Our permitted development guide covers the national rules.

Two real projects in WD6

Rear kitchen extension, Borehamwood. A 4 m deep rear extension across the back of the house, planned and built by us under one contract. The old kitchen became a generous utility room — the cupboard space and the washing-machine noise moved out of the living space — and the new extension took a large kitchen with a dining and social area and a snug. Planning was approved by Hertsmere and the project is complete; the project post has the full photograph set and the build video.

Inside the Borehamwood extension: the new kitchen under the rooflights, with the old kitchen behind it turned into a utility room.

Hip-to-gable loft conversion, Elstree. On a semi-detached house we turned the sloping hip of the roof into a vertical gable and added a rear dormer, from architectural design through structural steel to the finished bathroom, flooring, electrics and tiling. The project post shows every stage from the tin-hat scaffold to the finished room — and it is the same semi-detached house type that takes a double-storey rear extension well, which is why so many WD6 projects are a loft and an extension designed together.

Anderson's advice

“Rooflights over kitchen islands transform natural light more than any other single design decision.” The Borehamwood extension is 4 m deep across the full width of the house, which makes the old kitchen — now the utility — and the middle of the new room a long way from the garden glazing. The rooflights over the island are what keep the centre of the room bright at midday in October. Decide where the island goes first, then put the glass above it; it costs a few thousand pounds on a £50,000 – £60,000 extension and is the thing clients mention most afterwards.

Why design & build for a Borehamwood extension

As LABC Building Excellence Award Winners 2024 for Best Residential Extension in London — and London Regional Winners again in 2026 for Best Conversion or Alteration to an Existing Home — we handle the architectural design, structural engineering, Hertsmere planning and building control applications and the construction under one contract, with a 12-month workmanship guarantee and a 10-year LABC structural warranty. Building a new house in WD6 instead? See our new build houses Borehamwood guide.

What our clients say

“We used LCCL for our rear extension and were so impressed that we came back for a loft conversion. The quality of work and communication throughout was excellent.”

— Mia M, Google Reviews

Frequently asked questions

Do I need planning permission for an extension in Borehamwood?

Not for a single-storey rear extension of up to 3 m (semi) or 4 m (detached), or 6 m / 8 m with prior approval, on a house that still has its permitted development rights and is outside the Elstree conservation area. Double-storey extensions, and any extension on an estate where the original permission removed permitted development rights, need a householder application to Hertsmere.

How much does a rear extension cost in WD6?

£42,000 – £90,000 inc. VAT (£35,000 – £75,000 + VAT) in 2026 for 15–25 m² to a shell-and-plaster finish, with Hertfordshire postcodes in the lower half of the range — around £43,000 – £59,000 inc. VAT for a typical 20 m² extension before the kitchen, structural openings and fees.

Has LCCL built in Borehamwood before?

Yes — a 4 m rear kitchen extension in Borehamwood and a hip-to-gable loft conversion in Elstree, both designed and built by us under one contract, with project posts and photographs on this site.

Get a figure for your own project

To book a free 30-minute consultation call about your extension in Borehamwood, call LCCL Construction on 020 8037 7332 or get an instant estimate online — six questions, about two minutes, and you see a real range for your postcode on screen. Finchley N3 based, covering all of London and Hertfordshire.

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