How to Price a Garage Conversion: Labour Hours, Materials and Profit Margin

Quick Answer: A typical UK garage conversion in 2026 costs £18,000–£35,000 for a single integral garage and £25,000–£50,000 for a double or detached garage with full conversion to habitable space. The price depends heavily on what already exists: a well-built integral garage with sound floor, walls, and roof needs only insulation, glazing, and fit-out (lower end of range); a detached garage with single-leaf walls, no DPC, and no foundations of habitable load capacity needs reconstruction (higher end). Per-m² rates are typically £1,400–£2,200 for usable floor area. Always survey the existing fabric and structural drawings before quoting; assumptions about existing damp-proofing and foundations are the leading cause of cost overruns.

Summary

A garage conversion looks deceptively simple — there's already a structure, a roof, and (often) an existing wall in place — but the existing fabric rarely meets the standard required for habitable space. Garages were built to lower thermal, structural, and weather standards because they were designed for vehicles, not people. The conversion process is mostly upgrading what exists: insulating cold walls, lifting the floor to add a damp-proof membrane and insulation, fitting suitable windows and doors, and bringing the construction up to Approved Document standards across thermal, structural, and ventilation criteria.

For tradespeople, three garage conversion contexts dominate: integral garage (within the dwelling envelope, sharing walls with the house), attached garage (built against the house but with separate walls), and detached garage (separate building). Each has different cost drivers. Integral conversion is the cheapest because the structural envelope and connection to the house already meet building standards; detached conversion is the most expensive because the existing walls and roof rarely meet habitable standards.

The Building Regulations applications are unambiguous: garage conversion is a "material change of use" (Approved Document P, in which a non-residential space becomes residential). Full Building Regulations approval is required, including thermal upgrade to current standards (Approved Document L1B), fire separation (Approved Document B if connecting to dwelling), and structural sufficiency (Approved Document A if adding load to existing foundation).

Key Facts

Quick Reference Table

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Element % of total price (typical integral conversion) Typical £ for single garage
Floor build-up (DPM, insulation, screed) 12–18% £2,200–£4,500
Wall insulation and lining 14–20% £2,500–£5,000
Roof / ceiling insulation 6–10% £1,000–£2,200
Replacement windows / doors 8–14% £1,500–£3,500
Internal partitions and joinery 8–14% £1,500–£3,500
Plumbing first/second fix 6–10% £1,200–£2,500
Electrical first/second fix 5–9% £1,000–£2,200
Heating extension 4–8% £800–£1,800
Plaster and decoration 6–10% £1,200–£2,500
Externals (drainage, paving) 3–6% £600–£1,500
Overhead and profit 18–25% n/a

Detailed Guidance

Survey Before Quoting

The pricing of a garage conversion is dictated by what the survey reveals. A garage that looks fine externally may have:

The survey should include: floor level relative to existing house DPC; wall construction (single or double leaf, cavity or solid); roof construction; existing services (gas meter often relocated, electrics inadequate); and damp condition (any tide marks, salt staining, hygroscopic damage to internal face).

For integral and attached garages, the wall shared with the house is structural; check for any existing alterations (chases for cables, removed lintels) that may compromise the wall's load-bearing capacity once the garage door is removed and a window installed.

Building Regulations Application

Garage conversion is a "material change of use" under Building Regulations. Full Building Regulations approval is required (Building Notice or Full Plans submission). The approval covers:

Approved Document A (Structure) — confirmation that the existing structure can support the changed use; new lintels for window openings; any new partitions or ceiling alterations.

Approved Document B (Fire Safety) — for integral and attached conversions, fire separation between the new habitable room and the rest of the dwelling; if the garage connects to the house, the connection door must be fire-resistant.

Approved Document C (Resistance to Moisture) — DPM in the new floor; DPC continuity at the existing wall base; damp-proof tray below window cill; vapour control layers in new walls.

Approved Document E (Acoustic) — for any new partition between dwellings or living spaces.

Approved Document F (Ventilation) — extract or trickle vent provision per the new room use.

Approved Document L1B (Conservation of Fuel and Power) — U-values for thermal upgrade.

Approved Document P (Electrical Safety) — notification of new electrical work.

The full plans submission cost is typically £600–£1,000 for a typical garage conversion.

Floor Build-Up

Existing garage floors are commonly 100 mm of unreinforced concrete poured directly onto compacted hardcore, with no damp-proof membrane. A garage floor finish is typically 50–100 mm below the dwelling DPC.

To convert to habitable, the floor must include:

Two approaches:

Lift and rebuild — break out the existing slab, install new hardcore, sand blinding, DPM, insulation, screed. Most reliable but most expensive (£100–£150 per m²) and requires breakers, removal, and waste disposal.

Build-up over existing — apply DPM to existing slab, lay insulation board, screed or chipboard floating floor on top. Easier and cheaper (£60–£100 per m²) but raises the floor level by 80–150 mm, which may conflict with door heights, ceiling heights, or DPC continuity at the wall.

The choice depends on: existing slab condition (cracked or undulating slab argues for lift-and-rebuild), floor level relative to existing DPC, and headroom available.

Wall Insulation

Two main approaches:

Internal wall insulation (IWI) — PIR-bonded plasterboard or PIR + plasterboard built on a timber stud frame. Faster, cheaper, no external alteration. Key risk: dew-point migration in solid-walled garages (similar to the issue in IWI dew point migration in old-house retrofit). Specify a vapour control layer and check thermal bridging at the junctions with the existing dwelling.

External wall insulation (EWI) — EPS or mineral wool insulation faced with thin-coat render or brick slip. Better thermal performance, eliminates the dew-point risk, no internal space loss. More expensive (£130–£220 per m² versus £40–£75) and changes the external appearance — needs planning consideration in some areas.

For most domestic garage conversions, IWI is the cost-effective choice and is the standard specification. EWI is justified where the external aesthetic improvement matters or where the dwelling is in a conservation area requiring matching external finish.

Replacing the Garage Door with a Window

The most common visible alteration is replacing the garage door with a window. The opening typically reduces from 2.4 × 2.1 m (typical garage door) to 1.8 × 1.2 m (typical window) by infilling below the new cill and adjusting the lintel.

Sequence:

  1. Remove garage door and frame
  2. Install new lintel suited to the new window opening (existing lintel may be too long if not used as a structural span — seek SE confirmation)
  3. Build new masonry below the new cill height, matching the existing wall (new brick/block cavity wall, insulated, finished to match)
  4. Fit new window
  5. External make-good (render, brickwork, paint) to match existing
  6. Internal cill, reveal, and finish

The masonry-up portion typically costs £700–£1,500 in materials and labour. The window itself is £900–£2,000 for a standard PVC casement up to £2,500–£4,500 for a high-spec composite or aluminium frame.

Connection to Existing Services

Garage conversions typically need to extend or relocate services from the existing dwelling:

Heating — extend the existing central heating system. Check the existing boiler has spare capacity for the additional heat load (typical garage conversion adds 1–2 kW heat demand). New radiators £80–£160 each fitted, plus £100–£200 each for valves, isolators, and pipework. Underfloor heating is the alternative; works well in garages because the new floor build-up accommodates the heat-spreader plates.

Plumbing — for a kitchen, utility, or bathroom in the garage, run hot/cold supply and waste from the dwelling. Cost depends on routes and existing access.

Electrics — the existing garage circuit (typically a single 13A radial) is inadequate for habitable use. Run new circuits from the dwelling consumer unit: lighting circuit, ring final or radial sockets, dedicated circuits for kitchen appliances if relevant. £1,200–£2,500 typical for full new electrical fit-out, plus the consumer unit upgrade if needed.

Detached Garage Conversion — Additional Considerations

A detached garage is a separate building. Conversion to habitable use raises additional issues:

Structural adequacy — original detached garages were often built with strip foundations sized for garage live load (1 kN/m²) rather than habitable (1.5 kN/m²). For most well-built detached garages, the existing foundations are adequate; for borderline cases, structural engineer review is needed.

Fire separation — detached garage typically not subject to the same fire-separation requirements as integral, but if used as a granny annexe or guest room with overnight occupation, additional fire safety considerations apply.

Outbuildings versus dwellings — change of use from a detached garage (outbuilding) to a habitable dwelling (annexe or independent unit) often requires planning permission, even if internally only.

Drainage and connection — drainage to the existing dwelling soakaway or sewer requires excavation and connection.

Cost premium — detached conversion typically costs 30–50% more than integral conversion of the same floor area because of the additional structural, drainage, and electrical work.

Frequently Asked Questions

Do I need planning permission for a garage conversion?

Generally not for an integral or attached garage conversion that is purely internal — these usually fall within permitted development. Permission is needed for: detached garage conversions to dwellings or annexes, conservation areas with Article 4 directions, listed buildings, where the property's permitted development rights have been removed, and where the conversion involves external changes (replacing the garage door with masonry, for example, often requires a planning application even in PD-rights areas).

How much does a garage conversion cost in the UK?

Single integral garage: £18,000–£35,000 typical UK price 2026. Single attached garage: £22,000–£40,000. Single detached garage: £28,000–£50,000+. Double garage of any type: 60–90% more than single. London and south-east premium: 20–35%. The biggest variables are the floor build-up (lift vs build-up over), the wall insulation approach (IWI vs EWI), and the kitchen/bathroom inclusion. See the trade pricing methodology for cost markup approach.

How long does a garage conversion take?

Integral: 4–10 weeks site time. Attached: 5–12 weeks. Detached: 6–12 weeks. The variability is mostly in the floor build-up (lift-and-rebuild adds 1–2 weeks), in any party wall procedures (rare for integral, more common for attached), and in supply lead times for windows and bespoke joinery.

Will a garage conversion add value to my house?

Typically yes, by 5–10% of property value for a high-quality conversion to additional habitable space. The uplift is highest where the existing house has only 2–3 bedrooms and the garage conversion adds a bedroom, family room, or home office without removing parking value. The uplift is lowest where the house already has plenty of accommodation and the conversion just removes valuable garage parking — particularly in London where on-street parking is constrained.

Can I keep the garage door?

You can keep the garage door visually (keep it in place but install a new wall behind it for the habitable room), but the converted space behind must meet all the Approved Documents — the door is not the weather envelope. This "keep the garage door" approach is sometimes specified for character reasons or to preserve the option of a future garage conversion-back; it adds 5–10% to cost compared to removing the door fully.

Regulations & Standards