Annexe extensions in Harpenden: design, approvals and build
Our design and build teams take on annexe extensions in Harpenden, from the first measured survey through planning and building control to the finished room.
- Guide price in HarpendenFrom £58,000 inc VAT
- PlanningUsually a householder application
- Build time16–24 weeks on site
- Postcode districtsAL5
Planning and approvals in Harpenden
In England an annexe that stays part of the household and connects to the house is assessed as an extension, but its size usually takes it beyond permitted development, so a householder application describing the ancillary use is the normal route.
In a conservation area, national park, National Landscape, the Broads or a World Heritage Site the permitted development limits are tighter (no side or two-storey extensions without an application). Listed homes need listed building consent, and if permitted development rights have been removed, any extension needs an application.
Building over or within 3 m of a public sewer needs the water company's consent, usually through a build-over agreement, before the foundations go in.
Applications are decided by the local planning authority (your district, borough or unitary council).
Local to Harpenden
Site visits are available across Harpenden (AL5). Nearby, we also take on projects in St Albans (5 miles), Hemel Hempstead (6 miles), Welwyn Garden City (6 miles), Hatfield (7 miles).
Luton, in Bedfordshire, is roughly 5 miles from Harpenden and also on our list. The East of England mixes market-town terraces, inter-war semis and modern estates. Clay soils are common in parts of the region, which can affect foundation design.
What is included
- Separate or shared heating and hot water
- Fire separation and alarms to building regulations
- Planning application describing ancillary use
- Design for accessibility: level thresholds, wider doors, walk-in shower
- Kitchenette, shower room and bedroom finish
Guide prices for Harpenden
Labour and material costs vary across the UK, so for Harpenden we use our East of England factor (×1.06). The ranges include VAT and exclude design, structural and council fees.
Typical rates in Harpenden: £2,120 to £4,030 per m² including VAT, depending on specification.
| Size | Essential | Standard | Premium |
|---|---|---|---|
| Studio annexe 25 m² | £58,000 – £73,000 | £63,500 – £79,500 | £79,500 – £101,000 |
| One-bedroom annexe 35 m² | £74,000 – £93,000 | £89,000 – £111,500 | £111,000 – £141,000 |
| Larger annexe 45 m² | £95,000 – £119,500 | £114,000 – £143,500 | £143,000 – £181,500 |
For a figure based on your own dimensions, use our guide-price tool.
What happens next
Site visits are free and there is no obligation. Where drawings are needed, the design stage is priced in writing before it starts.
Free site visit
A designer visits, takes measurements and photos, and discusses layouts, budget and timing with you.
Design and approvals
We produce the drawings and structural design, and manage the approvals so they are in place before work starts.
Fixed written price
The price is fixed in writing once the design is settled, with a clear programme and payment stages.
Build and sign-off
Our team builds the extension, keeps you updated weekly and finishes with building control sign-off.
Annexe extension questions answered
Can I see a price before committing to anything?
Yes. The guide prices on this page are adjusted for Harpenden, and the guide-price tool gives a range for your own size. After the survey you get a written price before anything is agreed.
How long does an annexe extension take to build in Harpenden?
On site, typically 16 to 24 weeks once approvals are in place. Before that, allow 10–16 weeks design and approvals.
Who do I contract with for an annexe extension in Harpenden?
Your contract is with S&K Property Investments and Improvement Ltd, which designs and manages the project and the build teams who carry it out.
Will an annexe affect council tax?
It can. Some annexes are assessed separately for council tax, with discounts or exemptions in some cases when occupied by a relative. Check with the Valuation Office Agency (England and Wales), your local assessor (Scotland) or Land & Property Services (Northern Ireland).
Does an annexe extension need planning permission?
Usually yes, because of its size and the way it will be used. Councils check that it stays ancillary to the main house, so the application must describe the use clearly.
Book a free site visit in Harpenden
We will measure up, check the planning position and give you honest guide figures.
- Free, no-obligation site visit
- Planning route checked before you spend on drawings
- Written price once the design is agreed
- Building control sign-off at the end
Prefer to talk? Call 0800 088 6341 or .
Book a free site visit
Tell us a little about the project and we will arrange a time.