Detached House: Loft Conversion and Extension Under Permitted Development — Larger Limits, Fixed Price
By Anderson & the LCCL Construction Team | Finchley N3
Detached houses — the inter-war and post-war homes of Totteridge, Stanmore, Hadley Wood, Radlett and the Hertfordshire commuter towns, and the larger Edwardian villas of Hampstead Garden Suburb and Ealing — are the houses where permitted development stops being a constraint and becomes a generous design brief. The rules give a detached house more depth at the back, more volume in the roof and, with no party wall, a far simpler programme.
This guide works through what a detached house can take without a planning application and what each product costs at a fixed price from our permitted development price list. It assumes the house has its permitted development rights intact; large detached houses in conservation areas, on green belt edges or on estates with restrictive covenants are the usual exceptions, and we check all three before the design starts.
The roof: hip-to-gable on both sides
Most detached houses from the 1920s onwards have a hipped roof on both ends, which is why the loft usually feels big from the hatch and small on the floor. Permitted development allows up to 50 cubic metres of added roof volume on a detached house — the same as a semi, but with two side slopes to use. Building both hips up into gables and adding a flat-roofed rear dormer typically gives two double bedrooms and a bathroom, or a master suite with a dressing room, on a floor that was previously storage.
Our hip-to-gable with rear dormer is from £60,000 inc. VAT (£50,000 + VAT) on one side; a double hip-to-gable on a detached house is priced at survey because the structure, the roof covering and the floor area all roughly double. On a wide detached roof the 50 cubic metre allowance is the limit we design to, and where the brief genuinely needs more — a full-width dormer across a large rear slope, for instance — the same design goes through planning, which on a detached house in a residential street is normally a straightforward application. The Stanmore and Canons Park houses in our Stanmore loft guide are the typical case.
The back: 4 metres, or 8 metres with prior approval
A detached house can extend 4 metres from the original rear wall under permitted development, single storey and up to 4 metres high, and under the larger-home rules up to 8 metres with a prior approval notification — a 42-day neighbour consultation rather than a planning application. Eight metres is a very large room: on most detached houses it is the full kitchen, dining and family space in one, with rooflights down the middle and glazing across the back.
Our rear extension is from £42,000 inc. VAT (£35,000 + VAT) for the standard version; the larger prior-approval extension is priced at survey as the foundations, steels and roof scale with the depth. The variables on a detached house are the ground — larger plots in Hertfordshire often sit on clay, which decides the foundation depth — and the glazing, which on an 8 metre room is the single biggest line in the price.
The side: a side extension within half the width of the house
A single-storey side extension is permitted development on a detached house if it is no wider than half the width of the original house and no higher than 4 metres. On the typical detached plot with a drive down one side, that means a garage-sized room — a utility, a study, a second reception — without a planning application, at from £42,000 inc. VAT (£35,000 + VAT). The two combinations that are not permitted development are the same as on any house: a wraparound that joins the side and rear extensions into one footprint, and anything two storeys at the back, which is a double-storey extension at from £90,000 inc. VAT (£75,000 + VAT) via planning.
Why a detached house is the easiest build
No party wall means no party wall notices, no surveyor's award and no neighbour's consent in the critical path — on a semi or a terrace that alone can add three months to the start date. The scaffold goes up on your own land, the skip sits on your own drive and the loft staircase has more positions to choose from. Our loft and extension together product — hip-to-gable loft plus a rear extension under one contract — is from £98,400 inc. VAT (£82,000 + VAT), and on a detached house it is usually the shorter programme of the two house types.
The house this guide is built on
Our project at Lancaster Avenue in Hadley Wood is a 3,033 sq ft detached new build on exactly this kind of plot — the same ground, the same roof forms and the same planning authority as the detached houses we extend and convert across the Barnet and Hertfordshire border. The loft bedroom in the photograph above is from a hip-to-gable conversion on a 1930s house in East Barnet; on a detached house the same room is built on both sides.
Anderson's advice
Clients with detached houses almost always want to open up the ground floor at the same time as extending, and the thing they forget is the structural steel. Taking out an internal load-bearing wall to join the new extension to the old kitchen is a separate piece of structure — a longer steel on padstones, sometimes a post — and it is typically £4,000 to £12,000 on top of the extension itself. Decide at the design stage whether the house becomes open plan, not when the builder is on site, so the steels are designed and priced once.
Why detached-house owners choose LCCL
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 design the loft or extension, obtain the Lawful Development Certificate or prior approval, produce the structural drawings and build it with our own teams — one contract, one fixed price and one point of contact from the first survey to the completion certificate. Every project carries a 12-month workmanship guarantee and a 10-year LABC structural warranty. Already have an architect's drawings for your house? We price the build as drawn within seven working days.
What our clients say
“I highly recommend LCCL Construction. They completed our loft conversion to a very high standard, and the quality of workmanship throughout was excellent. The team was professional, reliable, and completed everything within a very reasonable timeframe. They were also fairly priced, which made the whole experience even better. We’re really pleased with the finished result and would definitely use them again.”
— Alina A, Google Reviews
Frequently asked questions
How far can a detached house extend without planning permission?
4 metres from the original rear wall as standard permitted development, or up to 8 metres with a prior approval notification to the council — a 42-day neighbour consultation, not a planning application. The extension must be single storey and no more than 4 metres high.
Can a detached house have a hip-to-gable loft on both sides without planning?
Yes, provided the total added roof volume stays within 50 cubic metres and the house is outside a conservation area or Article 4 area. On a large roof we design to that limit; if the brief needs more, the same design goes through a planning application, which we prepare.
What does a detached house extension cost?
Our fixed-price rear or side extension starts from £42,000 inc. VAT (£35,000 + VAT) for the standard permitted development size, complete with design, structural engineering, building control and finishes to economy specification. Larger prior-approval extensions and double hip-to-gable lofts are priced at survey; every figure is fixed in writing before you sign.
Get a figure for your own project
To book a free 30-minute consultation call about your detached house, 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.
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