Extensions over a garage across Harpenden
If you live in Harpenden and want more space without moving, an over-garage extension is one of the most practical ways to get it. We handle the drawings, approvals and the build.
- Guide price in HarpendenFrom £25,000 inc VAT
- PlanningHouseholder application needed
- Build time12–18 weeks on site
- Postcode districtsAL5
Planning and approvals in Harpenden
In England any side extension over one storey, including a room built over a side garage, needs a householder planning application. The fee is £548 from 1 April 2026 and councils aim to decide within 8 weeks.
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 regulations approval is separate from planning. In England you can use the council's building control team or a registered building control approver; a full plans application gets the drawings checked before work starts, and the inspector signs the work off with a completion certificate.
If the work involves a shared wall, building on the boundary, or digging foundations within 3 m (sometimes 6 m) of a neighbour's building, the Party Wall etc. Act 1996 applies and notice must be served before work starts: two months for work to a party wall, one month for new walls on the boundary and for excavations.
Applications are decided by the local planning authority (your district, borough or unitary council).
Covering Harpenden
Across AL5, we can visit, measure and advise. We also work in nearby 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.
Included in a typical project
- Strengthening or rebuilding the garage walls if needed
- Insulation, electrics, heating and finish
- Structural survey of garage walls and foundations
- New floor structure, walls and roof
- Opening into the existing first floor
- Design and householder planning application
Fitting the new room to the existing landing is the planning puzzle inside the house. We design the connection first, then the roof, so the extension does not end up with awkward steps or a corridor through a bedroom.
Over-garage extension prices in 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: £1,910 to £3,600 per m² including VAT, depending on specification.
| Size | Essential | Standard | Premium |
|---|---|---|---|
| Over a single garage 12 m² | £25,000 – £31,500 | £26,500 – £34,500 | £34,000 – £43,500 |
| Over a large single garage 16 m² | £30,500 – £37,500 | £35,500 – £46,000 | £45,500 – £58,000 |
| Over a double garage 24 m² | £45,500 – £56,000 | £53,000 – £69,000 | £68,500 – £86,500 |
For a figure based on your own dimensions, use our guide-price tool.
From first visit to completion
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
We visit your Harpenden home, measure up, talk through what you want and check the planning position.
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
Work is carried out to the agreed programme, inspected by building control and signed off at the end.
Over-garage extension questions answered
How long does an over-garage extension take to build in Harpenden?
On site, typically 12 to 18 weeks once approvals are in place. Before that, allow 8–14 weeks design and approvals.
Do you cover all of Harpenden?
Yes. We cover Harpenden and the surrounding parts of Hertfordshire. Site visits are free and there is no obligation.
Will my neighbours in Harpenden be involved?
If a planning application or prior approval is needed, the council notifies neighbours. If the work is on or near a shared boundary, party wall notices may also be needed.
Does building over a garage need planning permission?
In England and the rest of the UK, adding a storey over a side garage almost always needs a planning application, because two-storey side extensions are outside permitted development.
Can I keep using the garage during the build?
Usually not for parking. The garage roof is removed and the space below is used for access and props for part of the build.
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.