Garage Conversion vs Extension: Costs Compared

Compare the 2026 cost benchmarks fairly

The guide figures favour a garage conversion on headline cost, but the comparison is useful only when both routes describe the same finished result and VAT basis.

Checkatrade's garage conversion guide, updated in May 2026, gives £10,000 to £20,000 plus VAT for a typical 16 square metre integral or attached garage. It gives £10,000 to £14,000 for an integral garage and £15,000 to £25,000 for a detached garage. The guide assumes the structure is in reasonable condition and that insulation and screed can sit above the existing slab. Moving meter boxes, cutting new doorways and extra kitchen or bathroom fixtures are excluded.

Its house extension guide gives £1,800 to £3,000 per square metre excluding VAT for a full extension and £1,200 to £1,700 per square metre excluding VAT for a shell only extension. It also gives £40,000 to £56,000 excluding VAT for a 20 square metre standard extension. A shell does not provide the same finished result as a converted and decorated room, so it should not be used as the cheaper comparison without adding the missing services, glazing, internal work and finishes.

The Federation of Master Builders gives £7,000 to £32,000 for garage conversions and £1,800 to £3,000 per square metre for single-storey extensions. Its headline ranges do not state one VAT basis, so confirm tax treatment. These are benchmarks, not Belmax quotations.

Check the guidance: Checkatrade garage conversion cost guide, Checkatrade house extension cost guide, FMB garage conversion cost guide, FMB single-storey extension cost guide.

Keep the room specification, exclusions and VAT treatment beside every benchmark. None of these ranges is a Belmax price.

Bottom line: A garage conversion starts cheaper in the published examples, but compare complete rooms rather than headline totals.

Choose the route that can make the right room

The cheaper route is poor value if it creates a room that is too narrow, badly connected or unable to support its intended use.

Begin with the daily problem. A home office, snug, playroom or utility may fit comfortably inside an integral garage. A kitchen diner, accessible bedroom suite or large family room may need more width, daylight, garden connection or service space than the garage can provide. Measure the usable area after insulation, wall linings and any retained storage rather than relying on the external garage dimensions.

Trace how people will enter the new room. An integral garage may connect naturally to a hall, kitchen or utility, but forming a doorway through an external wall can require structural work and may leave awkward circulation. A detached garage can provide separation for work or hobbies, yet it may need longer routes for power, water, drainage and heating. Calling it a conversion does not remove those design decisions.

An extension can be sized and positioned around the required room, but the new area must still improve the existing plan. A rear extension can reduce garden depth and natural light to the original room. A side extension can affect access or neighbour relationships. Test furniture, doors, windows, circulation and the connection between old and new before comparing construction prices.

Bottom line: Choose the garage only if its finished dimensions and position suit the room, not simply because the structure exists.

Check whether the garage is genuinely convertible

Existing walls and a roof reduce the amount of new construction, but they must be suitable for a dry, insulated and safe habitable room.

Survey the slab, walls, roof and openings before setting the budget. The finished floor may need damp protection, insulation and a new build-up. External walls and the roof may need thermal upgrades. The former garage door opening needs a compliant infill, weathering and a suitable window or door. Low ceilings, damaged roofs or poor foundations can narrow the cost advantage.

The room use changes the technical work. An office or snug may mainly need heating, power, lighting and ventilation. A utility, bathroom or kitchen can add water, waste, drainage, extraction, waterproofing, fitted products and more electrical work. A bedroom needs careful attention to escape, ventilation, warmth, privacy and the relationship to the rest of the home.

Decide whether all or part of the garage should be converted. Keeping a front storage bay can protect space for bikes, tools or bins, but the new dividing wall, access and remaining garage function need design. The retained area must not compromise insulation, fire separation or the usable size of the new room.

Bottom line: Confirm the garage condition and intended room services before relying on the conversion benchmark.

Understand what an extension buys for the extra cost

An extension creates new floor area, preserves the garage and offers more layout freedom, but it adds groundworks, external structure, roof and wider making good.

An extension starts below ground with foundations and drainage checks, then adds external walls, insulation, a roof, windows and doors. It must connect structurally and thermally to the house, manage rainwater and make good the garden, paving and affected rooms. Large openings between the old and new spaces may need beams, temporary support and structural calculations.

That extra work can deliver a room that a garage cannot. The footprint can respond to the kitchen layout, dining table, accessible circulation, garden view or storage required. The garage can remain available for parking or household storage. The extension may also improve more than one existing room if the design resolves circulation, light and access across the whole ground floor.

More choice also creates more scope decisions. Roof form, glazing, kitchen or bathroom products, heating, flooring, decoration and landscaping can move the finished total far beyond a basic square-metre rate. For a Rugby project, drawings and a room schedule should define both the new construction and the alterations inside the existing house before quotations are compared.

Bottom line: Paying more for an extension can be justified when it creates the correct area and protects functions the garage still serves.

Count parking, storage and disruption

A conversion uses an existing asset, while an extension uses land and usually creates a longer, more disruptive construction programme.

List what currently lives in the garage. A car, bikes, tools, a freezer, bins and seasonal storage all need another place if the whole space becomes a room. A shed, drive alteration or new storage wall is part of the decision, even if it appears outside the conversion quote. Where on-plot parking is limited or protected by a planning condition, losing the garage may not be practical.

An extension keeps the garage but occupies part of the garden or side access. Groundworks, scaffolding, deliveries and weather-sensitive construction normally affect the household for longer. The work may also interrupt a kitchen or living room when the new opening is formed. Plan temporary cooking, storage, protection and access instead of treating disruption as cost-free.

Avoid generic claims about value uplift. The useful room, build quality, parking demand, storage, garden size and local buyer expectations all affect the outcome. If resale value is important, ask a local estate agent or valuer about the specific property and proposed plan. A national percentage is not evidence for one Rugby home.

Bottom line: Add the function you lose, the land you use and the disruption you create to the financial comparison.

Separate planning from building regulations

A garage conversion may avoid a planning application more often, but both routes normally need building-regulations attention and property-specific checks.

Planning Portal says planning permission is not usually required for a garage conversion when the work is internal and does not enlarge the building. However, permission may be needed for a separate dwelling, a listed building or material external changes. Permitted development rights or a planning condition may also restrict conversion, particularly on some newer developments or where the garage was required for parking.

An extension can fall within permitted development only when every relevant limit and condition is met. Planning Portal says previous extensions count towards the total enlargement and that the rules apply to houses, not flats or maisonettes. Position, height, boundary distance, materials and designated land can affect the route. Exceeding the limits usually means a householder planning application is needed.

Building regulations are separate. Planning Portal says converting a garage or part of one into habitable space will normally require approval, and most extensions also require approval. Rugby Borough Council advises checking whether permitted development rights have been removed, while Warwick Building Control operates the local authority building-control service. Confirm who prepares technical information, makes applications, books inspections and collects completion records.

Check the guidance: Planning Portal garage conversion planning guidance, Planning Portal garage conversion building regulations, Planning Portal extension planning guidance, Rugby Borough Council planning guidance.

Bottom line: Check the address and design under both systems before treating either route as approved.

Use five tests to make the decision

The right option is the lowest-cost route that creates the required room without sacrificing a function the household still needs.

First, test room fit. Draw the furniture, circulation, storage, windows and services inside the finished garage dimensions, then compare that result with an extension footprint. Second, test the whole home. Check the entrance to the room, daylight to adjoining spaces, garden access and whether the garage still has an important parking or storage role.

Third, test technical feasibility. Record the garage condition or, for an extension, the likely groundworks, drainage, structural opening and roof connection. Fourth, confirm planning, building control and any neighbour duties. Fifth, compare complete quotations using the same finish level, responsibilities and VAT basis.

Give each contractor the same postcode, photographs, dimensions, intended room, drawings and product expectations. Ask for design work, approvals, structure, insulation, windows, doors, heating, electrics, plumbing, drainage, plastering, flooring, decoration, external making good, waste and completion documents to be stated. Identify exclusions and provisional sums rather than filling gaps with assumptions.

For an initial conversation with Belmax, send the Rugby postcode, photographs of the garage and the relevant side or rear of the house, the room needed and any drawings or planning history already available. That information supports a useful next-step discussion. A project-specific quotation follows inspection and an agreed scope, not the national benchmarks in this guide.

• Room test: does the finished space fit the people, furniture and services required?
• Home test: what happens to parking, storage, garden, access and adjoining rooms?
• Technical test: what existing condition, groundworks, structure and service routes apply?
• Approval test: which planning, building-control and neighbour steps are needed?
• Quotation test: do both totals cover the same finish, responsibilities, exclusions and VAT?

Bottom line: Use one written brief and five tests before choosing the cheaper-looking route.