Double garage conversions
A double garage is 28–36m² — the floor area of two double bedrooms. Convert all of it, or convert half and keep one bay for storage. Both are good answers. They suit different houses.
Full conversion or half-conversion?
Both are proper answers. A full conversion turns all 28–36m² into living space, typically £18,000–£28,000+ at 2026 prices. A half-conversion turns one bay into a warm room of 13–17m² and keeps the other as a garage, typically £14,000–£19,000. Covenants, storage and parking decide which fits your house.
One room, one garage bay
- Rear or side section becomes a warm room of ~13–17m²
- The other bay keeps its garage door: bikes, tools, freezer, ladders
- New insulated partition between the two — full regs standard on the room side
- Often the cleaner answer where a covenant or tight street parking makes keeping garage space wise
- Typically £14,000 – £19,000
All 28–36m² working for the house
- One large open space, or split into two rooms with a stud partition
- Both openings infilled with matched brickwork and windows
- Room for the ambitious uses: annexe, big office, cinema room
- Typically £18,000 – £28,000+ depending on spec
| Factor | Half-conversion | Full conversion |
|---|---|---|
| Living space gained | 13–17m² | 28–36m² |
| Typical 2026 cost | £14,000 – £19,000 | £18,000 – £28,000+ |
| Time on site | 2–3 weeks | 3–5 weeks |
| Garage storage kept | One bay, behind its existing door | None |
| Parking covenant position | Often easier — parking provision can remain | Consent usually needed where a covenant applies |
| Later phase two possible | Yes — partition designed to come out | — |
The half-conversion is the most under-recommended option in this trade, and it's worth understanding why we push it more often than other firms. First, storage is real: if the garage genuinely holds things you need, a conversion that exports them to the spare bedroom hasn't added a room to your house. Second, it keeps options open — we build the partition so the second bay can be converted later as a phase two, reusing the same floor and wall approach. Third, on estates where deeds require garage parking to remain, converting one bay and keeping one often satisfies the covenant's purpose and makes developer consent conversations dramatically easier. We confirm your deed's exact wording before advising — the whole subject is unpacked in the estate covenants guide.
The uses a single garage can't hold
An annexe with a real en-suite
Thirty square metres is enough for a bed-sitting room, a proper shower room and a kettle-and-fridge corner — genuine independence for an older parent or an adult child saving for a deposit, without anyone living in a caravan-sized space. Sleeping accommodation brings the strictest regs requirements (escape windows, fire separation) and self-containment has planning implications, both covered honestly on our bedroom & annexe page.
An office for two — or an office and a meeting room
Two people on video calls in one room is a recipe for resignation letters. A converted double splits naturally into a two-desk office with an acoustic partition, or an office plus a small meeting/quiet room. Everything on our home office page — acoustic linings, wired data, heating options — applies, doubled.
A cinema or media room
The double garage is the best cinema-room candidate in most houses precisely because of what makes it bad at everything else: few windows, concrete floor, detached-feeling position. We build these with acoustic-insulated linings (mass and isolation, not just fluff), blackout as a design feature rather than an afterthought, a dedicated electrical circuit for the AV stack, conduit runs to the screen wall so no cable is ever visible, and ventilation sized for a room full of people with the door shut. The half-conversion version — cinema in one bay — is a popular compromise.
A gym that doesn't shake the house
Free weights over a floating floor need a different build-up: rubber-crumb matting over a screed rated for point loads, and isolation from the house wall if anyone deadlifts. Ventilation matters more than heating here — a gym generates its own heat and needs the moisture removed.
What a double costs: £18,000 – £28,000+
The span is wider than a single's because the specs diverge more — a dry open-plan room and an annexe with a shower room are very different projects sharing a footprint.
| Project | Typical range | On site |
|---|---|---|
| Half-conversion (one bay kept for storage) | £14,000 – £19,000 | 2–3 weeks |
| Full conversion — one open dry room | £18,000 – £24,000 | 3–4 weeks |
| Full conversion — two rooms | £20,000 – £26,000 | 3–5 weeks |
| Cinema / media room spec | £22,000 – £28,000 | 4–5 weeks |
| Annexe with en-suite | £26,000 – £34,000 | 5–7 weeks |
Why a double isn't just "two singles"
- The openings: two separate doors are infilled like two singles, but one wide door means the lintel or beam above must be checked — and sometimes replaced — to carry the new wall and window loads. That's an engineer's calculation, included in our price, not an on-site improvisation.
- Heating load: 30m² of new heated space is a real demand. We heat-check your boiler at the survey; where it's marginal, electric underfloor heating in part of the space avoids a boiler upgrade that would otherwise gatecrash the budget.
- More floor, same rules: membrane, insulation and screed across double the area is where economies of scale actually show up — per square metre, a double converts cheaper than a single (the 2026 cost guide shows the worked maths at £890/m²).
- Drainage strategy: wet rooms want to be at the house end of the garage where the drain runs are short. Planning this at design stage is the difference between a £900 connection and a £3,000 one.
- The roof: many doubles — especially detached and 1970s–80s attached ones — carry a flat roof, and a conversion is the moment to be honest about its condition. A sound flat roof gets insulated to 0.18 W/m²K as part of the works; one nearing the end of its life is far cheaper to re-cover now, while the scaffold and trades are already there, than two years after the room below is decorated. The survey tells you which yours is, and the quote prices whichever answer is true.
- Security of the retained bay (half-conversions): the new partition is built as a real insulated wall, so the storage bay becomes the "outside" — which means the door between them is an external-grade lockable door, and the retained garage door deserves a service while we're there. Your tools end up better protected than they were in the original garage, not worse.
Timeline for a full double
- Before we start (1–2 weeks)
Survey, covenant and PD checks, engineer's calculations for wide openings, fixed quote, building control submission, materials ordered.
- Week 1 — structure
Doors out, lintel work where needed, infill walls and windows in, floor membrane and insulation across the full footprint.
- Weeks 2–3 — services & linings
Partitions built, first-fix electrics, heating and any plumbing, insulation and boarding, skim. Wet-room tanking happens here on annexe jobs.
- Weeks 3–5 — finish
Flooring, second fix, sanitaryware on en-suite jobs, decoration, snagging, final building control inspection and completion certificate.
Double garage questions
How much does a double garage conversion cost?
From around £18,000 for a full conversion to one open dry room, up to £28,000+ for cinema-spec builds and beyond £30,000 for an annexe with en-suite. A half-conversion keeping one storage bay runs £14,000 – £19,000. All fixed quotes after survey.
Can I keep parking my car in one half?
Yes — that's the half-conversion, and the retained bay keeps its garage door and slab exactly as they are. Be realistic about measurements though: if the car doesn't fit the bay now (most modern cars don't fit modern bays), the retained half is really secure storage, which is still often worth keeping.
Does converting a double need planning permission?
The same rules as a single: internal conversion work is usually permitted development, with the usual exceptions (conservation areas, listed buildings, removed PD rights, covenants). One extra flag for doubles: creating a self-contained annexe can be treated as forming a separate dwelling, which is a planning matter regardless of PD — we cover this at survey and on the annexe page.
Can you do it in two phases?
Yes. We design half-conversions so the partition can come out later and the second bay can be converted as a phase two with the same floor and wall build-up. Two phases cost roughly 10–15% more than doing everything at once, but spread the spend and let you live with the first room before committing to the second.
Will a big open room feel like a garage?
Proportion is the risk with a full-width open space — a room 6m wide and 2.3m tall can feel like a hall. We manage it with ceiling design, window placement in both infills, and zoning (flooring changes, half-height dividers). Where the brief allows, two rooms or a room-plus-storage layout often lives better than one big rectangle.
Full or half? Get both prices
The free survey covers both options with a fixed quote for each — decide with real numbers, not guesses.