House Extensions in Hertfordshire: Fixed-Price Permitted Development Extensions from £42,000
By Anderson & the LCCL Construction Team | Finchley N3
Hertfordshire is where a large share of our extension work now goes. The county's housing — 1930s semis in Borehamwood and Potters Bar, Edwardian villas in St Albans and Harpenden, post-war detached houses in Radlett and Brookmans Park, Victorian terraces in Watford and Hertford — generally has more garden and more side space than the London equivalent, and most of it keeps its permitted development rights. That combination is what makes a fixed-price extension without a planning application possible.
This page is the hub for our Hertfordshire extension work. It covers what permitted development allows, our fixed prices, the councils you will deal with, and links to the town guides for Borehamwood, Watford, Hertford and Harpenden.
Permitted development on a Hertfordshire house
The rules are national. A semi-detached or terraced house can take a single-storey rear extension up to 3 metres deep without planning permission, or up to 6 metres with prior approval from the council; a detached house gets 4 metres and 8 metres respectively. A side extension can be single storey and up to half the width of the original house. Height is capped at 4 metres to the ridge and 3 metres to the eaves within two metres of a boundary. Our rear extension permitted development guide has the full checklist.
Hertfordshire has two complications London rarely does. First, Green Belt: a lot of the county's villages sit inside it, and while permitted development still applies to the house itself, anything needing a planning application faces a stricter test on size. Second, estate covenants and removed rights: many 1960s–1990s estates had permitted development withdrawn at the outset, so the deeds and the original planning decision need checking before anything is designed. We do both before the price is fixed.
Fixed prices across Hertfordshire
We price Hertfordshire exactly as we price London — from the same published table, with the figure fixed in writing before you sign. Headline prices for the complete job, including design, structural engineering, building control and finishes to economy specification:
3 m rear extension (4 m detached) — from £42,000 inc. VAT (£35,000 + VAT).
Single-storey side extension — from £42,000 inc. VAT (£35,000 + VAT).
Side-return infill on a terrace — from £42,000 inc. VAT (£35,000 + VAT).
Wraparound extension (normally a planning application) — from £72,000 inc. VAT (£60,000 + VAT).
Double-storey rear extension (planning application) — from £90,000 inc. VAT (£75,000 + VAT).
Loft and extension together — from £98,400 inc. VAT (£82,000 + VAT).
The final figure is postcode dependent and set at survey. Detached houses with deeper extensions, clay soils needing deeper foundations, and large glazed openings are the usual reasons a Hertfordshire job sits above the from-price. The instant estimate gives a range for your postcode in about two minutes; the fixed price follows the free consultation call and the survey.
Which council you will deal with
Hertfordshire has ten district and borough councils, and your Lawful Development Certificate or planning application goes to the one covering your address: Hertsmere (Borehamwood, Bushey, Radlett, Potters Bar), Watford, Three Rivers (Rickmansworth, Croxley Green), St Albans (St Albans, Harpenden), Dacorum (Hemel Hempstead, Berkhamsted), Welwyn Hatfield, East Herts (Hertford, Ware, Bishop's Stortford), Broxbourne, Stevenage and North Herts. The permitted development rules are identical in all of them; what varies is how quickly certificates are issued and how each handles prior approval. Our team deals with the submission and the correspondence so you do not have to.
Building control is separate from planning. We work with LABC — the local authority building control service — on every job, and the building regulations guide explains what gets inspected and when.
Our Hertfordshire-border projects
Bohun Grove in East Barnet, right on the Hertfordshire line, is the project that won us the LABC Building Excellence Award 2024 for Best Residential Extension in London: a 1930s semi with a rear extension opening the kitchen to the garden, a hip-to-gable loft above and a full renovation, all under one contract. At Lancaster Avenue in Hadley Wood we built a 3,033 sq ft new house from the ground up, which is why the detached-house extensions in Radlett, Brookmans Park and Harpenden hold no surprises for our team. And in Borehamwood WD6 a straightforward rear extension shows what the from-price buys when the drawings are complete.
For the company, projects and planning notes across the county, see our Hertfordshire area page.
Anderson's advice
August is the slowest month for starts, every year. Hertfordshire families almost always want the extension finished before the next school year, and the builders who can deliver that are booked by February. If you start the design in October or November, the Lawful Development Certificate and structural drawings are done over winter, the ground is opened in March and the kitchen is in by June. Leave the first call until Easter and you are competing for a summer slot with everyone else.
Why Hertfordshire homeowners 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 bring the same design and build team to Hertfordshire that we run in North London: one contract, one fixed price and one point of contact from the first survey to the completion certificate. Our Finchley N3 base puts most of the county within forty minutes, so project managers visit as often as they do in London. Every project carries a 12-month workmanship guarantee and a 10-year LABC structural warranty. If your architect has already drawn the extension, 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
Which parts of Hertfordshire do you cover?
The whole southern and central county: Borehamwood, Elstree, Radlett, Potters Bar, Bushey, Watford, Rickmansworth, St Albans, Harpenden, Hatfield, Welwyn Garden City, Hertford, Ware, Hemel Hempstead and Berkhamsted, plus the villages between them. Our base is Finchley N3, so most Hertfordshire sites are within forty minutes.
Does Green Belt stop me extending my house in Hertfordshire?
Not under permitted development — the national allowances still apply to the house itself. Green Belt matters when an extension needs a planning application, where councils apply a size test against the original house. We check the designation and the planning history before the design starts.
How much does a house extension cost in Hertfordshire?
Our fixed-price single-storey rear, side and side-return extensions start from £42,000 inc. VAT (£35,000 + VAT) for the complete job. Wraparounds start from £72,000 and double-storey extensions from £90,000 inc. VAT. The exact figure is postcode dependent and fixed in writing after the survey.
Get a figure for your own project
To book a free 30-minute consultation call about your Hertfordshire extension, 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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