Granny Annexe Costs UK (2026): What You'll Really Pay

Ask five suppliers what an annexe costs and you'll get five confident answers, none of which include the groundworks. This guide gives you the full picture: what each build route really costs in 2026, the line items headline prices leave out, and how to sanity-check a quote.

It's written for families budgeting between roughly £20,000 and £160,000, which covers almost every annexe built in the UK.

Cost by build route

Prefab and modular annexes: £20,000-£70,000

Factory-built annexes are the value benchmark. Entry-level studios start around £20,000-£35,000; well-specified one-bedroom modular annexes run £40,000-£70,000 installed. The factory price is usually genuinely fixed - the variability lives in your site, not the building.

Speed is the other advantage: manufacture runs in parallel with planning, and installation takes days once the base is in. See our prefab and modular annexe guide for how to compare specifications.

Garage conversions: £15,000-£45,000

The cheapest route when the structure cooperates. A structurally sound garage converts to a studio for £15,000-£30,000; adding a bathroom and kitchenette typically takes the total to £25,000-£45,000.

The trap is a mediocre garage: marginal head height, poor foundations or damp can absorb £15,000 before any fit-out starts. Our honest test - if the conversion quote passes £40,000, price a modular garden annexe before signing anything. More in the garage conversion guide.

Brick-built garden annexes: £60,000-£160,000+

Traditional construction costs more and takes months, but matches the main house, tends to satisfy planners in sensitive locations, and reads as permanent value to surveyors. Expect £60,000-£120,000 for a studio and £80,000-£160,000+ for a one-bedroom build at a quality specification.

Annexe extensions: £70,000-£140,000

Attached annexes price like extensions: roughly £2,200-£3,500 per square metre depending on region and specification, so a 30-40 sqm single-storey annexe wing typically lands between £70,000 and £140,000 including bathroom and kitchenette. What the premium buys is integration - care a corridor away, shared services, and the strongest resale story of any route. See annexe extensions.

The costs headline prices leave out

This is where budgets go wrong. On top of the build price, allow for:

• Groundworks and base: £3,000-£10,000 for a modular annexe depending on levels, access and soil
• Service connections: £2,000-£8,000 for water, drainage and electricity runs; drainage is usually the constraint that shapes the whole design
• Planning costs: application fee of a few hundred pounds, plus £1,000-£2,500 if a consultant is needed (when permission is needed)
• Surveys: tree, drainage or ecology surveys where the site demands them, £300-£1,500
• Building regulations: inspection and certification, often bundled by good suppliers - confirm it in writing
• Furnishing and equipping: realistically £3,000-£10,000 for a home someone will actually live in

A fair rule of thumb: add 15-25% to any modular headline price for a real, site-specific total. For traditional builds, carry a 10-15% contingency on top of the contract price.

Accessible design: the cheapest money you'll spend

Level thresholds, a wet room instead of a shower tray, doorways at accessible widths and grab-rail-ready walls add a few hundred to a few thousand pounds at build time. Retrofitting the same features after a fall costs multiples of that, and usually happens at the worst possible moment. If the annexe is for an older relative, specify these from day one.

The comparison that matters: care costs

Residential care in the UK averages £40,000-£60,000 per year, and nursing care more (Age UK has current figures). Against that, even a £90,000 annexe is repaid in under two years of deferred care fees - and the family keeps the building.

An annexe is not a care home substitute where professional care is genuinely needed. But for the long stretch where a parent mainly needs proximity, safety and company, the economics are not close.

Sanity-checking a quote

Ask every supplier the same questions and compare answers side by side:

1. Is the price fixed, and does it include groundworks, connections and building-regs sign-off?
2. Is the planning application included, and what happens to my deposit if permission is refused?
3. What are the floor and wall U-values, and what warranty backs the structure? (10-year structural cover is the norm from reputable manufacturers)
4. Can I visit a completed annexe that a family actually lives in?
5. How are payments staged? Be wary of any schedule that is majority-paid before anything arrives on site.

Two suppliers £20,000 apart are usually quoting different buildings, not different margins. The specification sheet, not the brochure, tells you which.

What to do next

Start by confirming your planning route - it costs the least and de-risks the most: annexe planning permission explained. Then check what the annexe means for your bills in annexe council tax rules, and how families fund the build in paying for an annexe.

When you're ready for numbers specific to your garden, tell us about your project and we'll point you at suppliers suited to your budget and route.

Costs reflect the UK market in mid-2026 and vary by region and specification. Figures are for guidance, not quotations.