How Much Does an Extension Over a Garage Cost?

Quick Cost Overview

• Extension over a single garage costs range from £17,000 to £38,000 on average.
• A structural survey from £350 to £1,000 is almost always required before work begins.
• Most above-garage extensions require planning permission, as they alter the height and external appearance of the property.
• Labour typically accounts for around 50 to 60% of the total project cost.

Extension Over a Garage Cost Guide Contents

1. Cost Overview: Extension Over a Garage
2. What Affects the Cost of Building Over a Garage?
3. Garage Extension Structural Cost Considerations
4. Planning Permission for an Extension Over a Garage
5. What Can Your Garage Extension Space Be Used For?
6. Garage Extension Labour and Material Costs
7. Additional Garage Extension Costs to Budget For
8. Find a Garage Extension Builder on MyBuilder
9. FAQs: Common Questions About Extensions Over a Garage

Cost Overview: Extension Over a Garage

The cost of an extension over a garage varies considerably depending on the size of the garage, the specification of the build, and your location in the UK.

As a general guide, the average cost per square metre falls between £1,250 and £3,000, with a typical 20m² extension costing between £25,000 and £60,000 all in.

The table below gives a useful starting point for budgeting across different scenarios:

Project Type	Budget
Over a single garage	£17,000 - £23,000
Over a double garage	£34,000 - £40,000
Single garage with en suite	£20,000 - £26,000

Budget-end figures typically reflect a straightforward build with standard finishes. Mid-range and high-end costs reflect more complex structural requirements, higher-spec materials, bespoke glazing, and premium internal fit-outs.

For context on how these costs compare to other extension types, see our guide to bedroom extension costs.

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What Affects the Cost of Building Over a Garage?

The cost table above gives you a useful starting point, but the real price of your project will be shaped by the specifics of your garage and what you want to do with the space. Some of these variables are within your control, others, like the structural condition of the existing garage, you will not know until a survey has been done. Here is what to look out for.

• Size of the extension: A double garage gives you substantially more floor space than a single, but the cost per m² often decreases slightly at scale. The added space also opens up more possibilities for how the room is used.

• Structural condition of the garage: This is the most significant variable. If the existing garage walls and foundations are strong enough to support an additional storey, costs are more predictable. If reinforcement or underpinning is needed, costs can rise considerably.

• Specification and finishes: Standard finishes keep costs closer to the budget end. Bespoke windows, premium flooring, underfloor heating, and high-end bathroom fittings push costs toward the high-end range.

• Complexity of design: A simple pitched roof extension is considerably cheaper than a design with dormers, large glazed sections, or complex rooflines.

• Location: Labour rates in London and the South East typically run 20 to 30% higher than national averages. Urban areas may also add costs for parking, access, and skip hire.

• Existing roof condition: Building over a garage often highlights how worn the existing house roof is by comparison. Many homeowners take the opportunity to retile or re-slate the main roof at the same time - worth factoring into the overall budget if yours is due.

Garage Extension Structural Cost Considerations

Before any above-garage extension can be built, a structural engineer needs to assess whether the existing garage structure can support the additional load.

Many garages, particularly older detached ones, have foundations and walls that were built to carry only a single-storey structure and will need reinforcement before work can begin.

The cost of structural reinforcement varies widely depending on what is needed, but underpinning a garage can add £5,000 to £15,000 or more to the project.

A structural survey typically costs £350 to £1,000 for a basic residential assessment. This should always be arranged before detailed planning or quoting begins, it is the survey that determines whether the project is straightforward or complex, and therefore what the realistic budget looks like.

Key structural checks a surveyor will assess include:

• Whether the existing garage walls can bear the load of a new storey

• The depth and condition of existing foundations

• Whether the current roof structure needs full removal or can be modified

• Whether any party wall issues apply if the garage adjoins a neighbour's property

Note: If the garage shares a wall with a neighbouring property, a Party Wall Agreement may be required before work can begin. Party wall surveyor fees typically run £700 to £1,000 per surveyor.

Planning Permission for an Extension Over a Garage

Unlike many single-storey rear extensions, most above-garage extensions require full planning permission rather than falling under permitted development. This is because they alter the height of the building and its external appearance, which typically takes them outside permitted development limits.

Planning Permission Costs:

The current planning permission fee for a householder application in England is £528. Applications typically take around eight weeks to be determined, so this needs to be factored into your project timeline from the outset.

Even where permitted development might technically apply, above-garage extensions often affect the front or side elevation of the property, which is more tightly controlled than rear extensions. Always check with your local planning authority before assuming permitted development rights apply.

Building regulations costs:

Building regulations approval is required regardless of whether planning permission is needed. For a double-storey or above-garage extension, building regulations costs typically fall between £750 and £900 for a building notice, plus £370 to £470 for inspections. A structural engineer will need to provide structural calculations as part of the building regulations submission.

If you live in a conservation area, listed building, or an area where Article 4 Directions have removed permitted development rights, planning requirements will be more stringent. Always confirm your planning position before commissioning detailed drawings.

What Can Your Garage Extension Space Be Used For?

One of the more practical considerations when planning an above-garage extension is what the additional space will actually be used for, and whether the size of the garage makes it suitable for that purpose.

An extension over a single garage typically adds around 15 to 20m² of usable floor space. This is enough for:

• A single bedroom or child's bedroom
• A home office or study
• A small en suite bathroom for an existing bedroom

An extension over a double garage adds substantially more space, typically 25 to 40m², and opens up more ambitious options:

• A master bedroom with en suite
• A large home office or studio
• A family bathroom plus additional bedroom
• A self-contained annexe (subject to planning)

It is worth noting that adding an en suite is one of the most popular uses of this space, but it is also one of the most expensive elements of the fit-out. A fully fitted bathroom typically adds £4,500 to £7,000 to the project on top of the core extension cost, depending on the specification. If plumbing runs are long or complex, this can increase further.

Garage Extension Labour and Material Costs

Labour typically accounts for 50 to 60% of the total cost of an above-garage extension. For most projects, you will need a main contractor or builder coordinating several trades, including bricklayers, roofers, electricians, and plumbers.

Labour Cost

Typical hourly rates for the key trades involved are:

• Builders / bricklayers: £25 to £50 per hour
• Electricians: £30 to £45 per hour - see our electrician hourly rate guide for more detail
• Plumbers: £35 to £60 per hour - see our plumber cost guide for more detail

Material Cost

On the materials side, the main cost components are:

Material	Typical Cost
Bricks	£800 - £1,200 per 1,000
Structural reinforcement (if needed)	£5,000 - £15,000
Insulation and plastering	£20 - £30 per m²
Windows	£500 - £1,000 per window
Roofing	£50 - £100 per m²

Most builders will quote on a full supply-and-fit basis for the structural elements, with materials included in the overall price. Always ask for an itemised quote so you can compare like for like between contractors.

On MyBuilder, you can post your job with as much or as little detail as you have, browse profiles of local builders who have completed similar above-garage projects, and read reviews from homeowners before deciding who to invite for a quote.

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Additional Garage Extension Costs to Budget For

Beyond the core build, several additional expenses shape the final cost of an above-garage extension. These are worth confirming with your builder before work starts rather than discovering them mid-project.

Architectural and Design Fees

Typically run between 5% and 15% of the total project cost, depending on the scope of service required.

Structural Engineer Fees

A structural engineer is needed for the survey and building regulations calculations. Budget £350 to £1,000 for the initial assessment, with further fees for technical drawings on top.

Utilities

Extending electrics, plumbing, and heating into the new space typically adds £500 to £2,000 depending on the complexity of the runs.

Interior Finishes

Flooring, lighting, and decoration can add a further £5,000 to £10,000 for a well-finished room - and are rarely included in a builder's core quote.

Roof Replacement

If an existing flat roof needs to be removed and replaced with a pitched roof, budget £1,000 to £5,000 for site preparation and roofing work. A new pitched roof is often recommended anyway, as it improves insulation and tends to sit more naturally alongside the main house.

Contingency

Keep back 10 to 15% of your total budget for unexpected findings once work is underway - structural surprises, services diversions, and ground conditions are all common sources of additional cost on this type of project.

For finishing touches, see our guide to painter and decorator costs for an idea of what redecoration will add to the overall spend.

Find a Garage Extension Builder on MyBuilder

It is easy to find a local builder for your above-garage extension on MyBuilder. Post your job and available builders in your area can respond with their services and pricing. You can browse their profiles, read reviews from local homeowners, and see photos of previous extension work before making a decision.

All tradespeople on MyBuilder undergo checks at registration - such as ID documents, company details, certifications for regulated jobs and skill assessments - allowing you to hire with confidence.

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FAQs: Common Questions About Extensions Over a Garage

Do I Need Planning Permission to Build Over a Garage?

In most cases, yes. Above-garage extensions alter the height and external appearance of the property, which typically takes them outside permitted development limits.

The current planning permission fee in England is £528, and applications usually take around eight weeks to determine. Always check with your local planning authority before assuming permitted development rights apply to your project.

How Long Does an Extension Over a Garage Take to Build?

Most above-garage extensions take between 10 and 14 weeks from start to finish, including the design phase, obtaining permits, and construction. Simpler projects with no structural complications may be completed faster, while those requiring planning permission, party wall agreements, or structural reinforcement will take longer. Factor in additional lead time before work begins for planning (typically eight weeks) and the party wall notice period (at least two months) if applicable.

Can an Above-Garage Extension Increase My Home's Value?

Yes. Adding usable floor space through an above-garage extension is generally considered a positive by buyers, particularly if the new room is a bedroom, home office, or bathroom. Extensions can add up to 20% to a property's value, though the exact uplift depends on location, quality of the build, and current market conditions.

You can browse photos of completed extension projects on MyBuilder to see how local builders have approached similar jobs and what different specifications look like in practice.

Will an Extension Over a Garage Affect My Home Insurance?

Yes, in most cases. Adding a new habitable room increases the rebuild value of your property, which means your buildings insurance sum insured will need to be updated. Failing to notify your insurer before or after the build could leave you underinsured in the event of a claim.

Is It Cheaper to Build Over a Garage Than to Build a Standard Extension?

Generally, yes. Though it depends on the structural condition of the existing garage. Because the groundworks and foundations are already in place, you avoid one of the most expensive phases of a standard extension build. However, if the garage requires structural reinforcement or underpinning before it can support an additional storey, those savings can quickly be eroded.