Extensions over a garage in Dorchester
Our extensions over a garage service in Dorchester covers everything: measured survey, design, planning or permitted development checks, structural design, building control and the build itself.
Dorchester: the key facts for this kind of project.
- Postcode districts
- DT1 and DT2
- County / area
- Dorset, England
- Price region
- South West England
- Guide price
- £32,500–£47,500 (£1,800–£2,630 per m²)
- Approval route
- Planning permission is needed: in England, permitted development only allows single-storey extensions to the side.
- Nearest towns
- Weymouth (7.2 miles), Portland (10.3 miles), Bridport (14.3 miles)
- Distance to Bournemouth
- about 24.2 miles
- First visit
- Free, no obligation
About an extension over a garage
An extension over a garage suits you if you have an attached single garage beside the house and need an extra bedroom or a larger main bedroom with an en-suite. You get an inspection of the garage structure, a design that matches the house, the approvals your nation needs, structural design, building regulations approval and the build, carried out by vetted teams working under S&K.
Around Dorchester we also work in Weymouth, Portland and Bridport, and across the rest of Dorset. Postcode districts in and near Dorchester: DT1 and DT2.
What it costs in Dorchester
Most of the cost of an extension over a garage is set by its size and specification. For Dorchester and the rest of South West England, our guide is £1,800–£2,630 per m² including VAT, so about £32,500–£47,500 for 3 m × 6 m of new first-floor space (18 m²). Design fees, structural calculations and approval fees are separate and listed on our prices page.
| Spec | Per m² | 3 m × 6 m of new first-floor space (18 m²) |
|---|---|---|
| Essential | £1,660–£2,420 per m² | £30,000–£43,500 |
| Standard | £1,800–£2,630 per m² | £32,500–£47,500 |
| Premium | £2,160–£3,150 per m² | £39,000–£56,500 |
Planning and building regulations in Dorchester
In England, most garages sit beside the house, so building over one makes a two-storey extension beyond a side wall, which always needs planning permission. If the garage is at the rear, the 3 m and 7 m rules for two-storey rear extensions apply, measured from the original house. A householder application costs £548 (or £575 for applications made from 8 December 2026).
Approval in England: Householder planning application: £548 (or £575 for applications made from 8 December 2026). Lawful development certificate: £274 (about £288 for applications from 8 December 2026). Councils should decide householder applications within 8 weeks of validation.
Building regulations: Building regulations approval (full plans recommended) from the council or a registered building control approver, with a completion certificate at the end.
Design points for Dorchester homes
Fire separation
If the garage stays a garage, the floor between it and the room above, and any door into the house, must give fire separation under building regulations. That usually means fire-resisting plasterboard ceilings, sealed gaps and a fire door, which we design in from the start.
Checking the garage foundations
Garages are often built on shallow strip footings sized for a single-storey wall and a light roof. A trial hole shows their depth and condition, and the engineer decides whether they can carry two storeys or need underpinning or new foundations inside the garage.
How we'd run your project
Start at your house
We come to your home in Dorchester, measure up, check the ground floor walls and roof structure, and note anything near the boundaries.
Drawings and permission
Our designers produce plans and elevations, then we take care of planning or permitted development checks with the council covering Dorchester.
Price it properly
Once the engineer's calculations are in, we fix the build price in writing, with a clear list of what's included.
Build
Foundations, walls, floor, roof, windows, then first and second fix. We break through into the house as late as we can.
Questions about extensions over a garage in Dorchester
Do I need planning permission for an extension over a garage in Dorchester?
Dorchester is in England, so England planning rules apply. Planning permission is needed: in England, permitted development only allows single-storey extensions to the side. If in doubt, a lawful development certificate or a planning application gives you certainty before you build.
Which postcodes do you cover around Dorchester?
We cover Dorchester (DT1 and DT2) and nearby places including Weymouth, Portland and Bridport, plus the rest of Dorset. The first site visit is free.
How long does an extension over a garage take?
Allow 4 to 8 weeks for survey and design, then the approval period (councils should decide householder applications within 8 weeks of validation), then 4 to 8 weeks of technical design, which can overlap. The build itself usually takes 14 to 20 weeks, longer for large or complex designs.
Can my garage take a room on top?
Sometimes, but it needs checking. Many garages are built on shallow footings with thin walls, designed only for a single storey. We dig a trial hole and inspect the walls, and the structural engineer decides whether to reuse them, strengthen them, underpin the footings or build new foundations inside the garage.
Do I need to convert the garage as well?
No. You can keep the garage for parking or storage and build only the room above, as long as the floor between them and any door into the house meet fire separation rules. Converting the garage at the same time is often good value, because the walls and foundations may be upgraded anyway.
Nearby
We also cover places near Dorchester, including:
Other extensions in Dorchester
- Rear
- Side
- Wraparound
- Part two-storey
- First floor
- Semi-detached
- Detached
- Kitchen and bedrooms
- Bedroom and en-suite
All two-storey extension services in Dorchester · Extensions over a garage across Dorset
Ready to talk it through? Call us or send the form and we'll arrange a visit to your home in Dorchester.