How Much Does It Cost to Build a Granny Annexe in 2026?

How much to build a granny annexe is the first question most families ask, and the answer is frustratingly broad. You will find prices ranging from £30,000 for a basic garage conversion to over £200,000 for a luxury two-bedroom brick build. The real question is not 'how much does it cost?' but 'how much does it cost to build one properly?' This guide breaks down every cost element, compares different build methods (including the popular option of converting a double garage to a granny flat), and explains how to get genuine value for your money. At Grannexe, we have been building annexes across Kent, London, and the South East for over 20 years, so our figures are based on real projects, not estimates.

The Quick Answer: How Much to Build a Granny Annexe

Before we dive into the detail, here are the headline figures for 2026. A purpose-built studio granny annexe from a quality builder costs between £80,000 and £120,000. A one-bedroom annexe ranges from £100,000 to £160,000. A two-bedroom annexe costs between £120,000 and £220,000. These are all-in figures for a fully finished, habitable building with kitchen, bathroom, heating, insulation, utility connections, and basic landscaping. At Grannexe specifically, our prices start from £90,000 for a studio, £110,000 for a one bedroom, and £130,000 for a two bedroom. All Grannexe prices are zero VAT under the Caravan Act, which makes a significant difference compared to builders who charge 20 percent VAT on top. A £130,000 annexe from a builder charging VAT would actually cost £156,000. The same quality from Grannexe costs £130,000, full stop.

Studio annexe: £80,000 to £120,000 (from £90,000 at Grannexe, zero VAT)
One bedroom annexe: £100,000 to £160,000 (from £110,000 at Grannexe, zero VAT)
Two bedroom annexe: £120,000 to £220,000 (from £130,000 at Grannexe, zero VAT)
Budget builders may quote less but often exclude essential costs
Always compare total costs including VAT

Cost to Convert a Double Garage to a Granny Flat UK

Converting a double garage to a granny flat is one of the more affordable routes to creating self-contained accommodation. The cost to convert a double garage to a granny flat in the UK typically ranges from £30,000 to £70,000, depending on the current condition of the garage, the standard of finish, and whether structural work is needed. A basic conversion (insulation, plasterboard, simple kitchen and bathroom, basic flooring) can be done for £30,000 to £40,000. A mid-range conversion with proper insulation, a good kitchen, tiled bathroom, underfloor heating, and quality flooring costs £40,000 to £55,000. A high-end conversion with full building regulations compliance, high-spec kitchen, walk-in shower, and premium finishes can reach £55,000 to £70,000. However, there are important caveats. A garage conversion has inherent limitations. The floor level is usually lower than the house, which can create damp issues. The roof structure may not support proper insulation without significant modifications. The space is fixed by the existing structure, so the layout may be compromised. And a converted garage rarely achieves the same insulation performance as a purpose-built annexe.

Basic garage conversion: £30,000 to £40,000
Mid-range conversion: £40,000 to £55,000
High-end conversion: £55,000 to £70,000
Floor level and damp are common challenges
Roof structure may limit insulation options
Layout constrained by existing garage dimensions
Planning permission may be required for change of use
Building regulations approval needed for habitable space
Does not qualify for zero VAT under the Caravan Act

Garage Conversion vs Purpose-Built: Which Is Better Value?

On the surface, a garage conversion looks like the cheaper option. And in absolute terms, it is. But value is about what you get for your money, not just the price. A mid-range garage conversion at £50,000 gives you a space constrained by existing walls, potentially compromised insulation, and a layout that was designed for parking cars, not comfortable living. You lose your garage (and the storage and parking it provided), and you may need to budget for driveway alterations. A purpose-built modular annexe at £90,000 to £110,000 gives you a fully designed living space with optimal room proportions, 250mm WarmWall insulation, a proper kitchen and bathroom, accessibility features as standard, and zero VAT. You keep your garage, the annexe is positioned in the best location in your garden, and the build quality is controlled from start to finish. The price difference between a good garage conversion (£50,000 to £60,000) and a Grannexe studio (from £90,000) is £30,000 to £40,000. For that additional investment, you get a dramatically better building that will last longer, perform better thermally, and add more value to your property.

Garage conversion: lower absolute cost but constrained by existing structure
Purpose-built annexe: higher cost but optimised for comfortable living
Garage conversion loses your parking and storage space
Purpose-built annexe preserves the garage and is positioned optimally
Insulation performance: purpose-built exceeds garage conversion
Property value uplift is typically greater with a purpose-built annexe
The £30,000 to £40,000 premium for purpose-built is usually worth it long term

Site Preparation Costs

Site preparation is one of the most variable cost elements and one of the areas where budget quotes often fall short. Site prep includes clearing the area, removing existing structures (sheds, decking, trees), excavating for foundations, levelling the ground, and creating access for construction vehicles. For a straightforward site with flat ground and easy access, site preparation typically costs £5,000 to £10,000. This covers basic groundwork, foundation preparation, and removing minor obstacles. For more challenging sites with slopes, poor ground conditions, trees with preservation orders nearby, or limited access, costs can rise to £10,000 to £25,000 or more. Deep foundations, retaining walls, or specialist drainage can add significantly to the bill. At Grannexe, our standard pricing includes site preparation for straightforward sites. We conduct a thorough site survey before quoting, so if additional groundwork is needed, we tell you upfront rather than presenting unexpected costs midway through the build.

Straightforward site: £5,000 to £10,000
Sloping ground requiring levelling: £8,000 to £15,000
Poor ground needing deep foundations: £10,000 to £20,000
Tree removal or protection measures: £1,000 to £5,000
Access creation for construction vehicles: £2,000 to £5,000
Grannexe includes standard site prep in the quoted price

Utility Connection Costs

A self-contained granny annexe needs electricity, water supply, drainage, and usually a gas or electric heating system. The cost of connecting these utilities from the main house to the annexe depends primarily on the distance between the two buildings. Electricity is the most straightforward connection. Running a dedicated supply from your consumer unit to the annexe typically costs £1,500 to £3,500, depending on distance and whether the cable route needs to cross hard surfaces. Water supply involves running a pipe from the mains supply or the main house to the annexe. Costs range from £1,000 to £3,000. Drainage can be connected to the existing household drainage if the annexe is close enough. If not, a separate drainage run to the main sewer is needed, which can cost £2,000 to £6,000 depending on the distance and ground conditions. Heating is usually electric (underfloor heating or panel heaters) in modern modular annexes, as this avoids the need for a gas connection. Electric heating is efficient in well-insulated buildings and keeps installation costs lower. Some clients prefer an air-source heat pump, which adds £3,000 to £6,000 but reduces running costs further.

Electricity supply: £1,500 to £3,500
Water supply: £1,000 to £3,000
Drainage connection: £2,000 to £6,000
Electric heating (underfloor or panels): included in build cost
Air-source heat pump (optional): £3,000 to £6,000 extra
Total utility costs: typically £5,000 to £12,000
Distance from main house is the biggest variable

The VAT Question: Why It Matters So Much

VAT is one of the single biggest factors in the total cost of a granny annexe, yet many families do not consider it until they receive the final invoice. Brick-built annexes and many timber frame builds are subject to standard rate VAT at 20 percent. This is applied to the entire build cost, including materials, labour, and the builder's margin. On a £120,000 build, that is £24,000 in VAT alone. Annexes built under the Caravan Act 1960 are zero-rated for VAT. This is not a reduced rate or an exemption. It means VAT is charged at 0 percent. The saving is real and significant. Every Grannexe annexe is built to comply with the Caravan Act, which requires the structure to be no larger than 20m x 6.8m and transportable in no more than two sections. Despite this technical classification, our annexes are indistinguishable from permanent buildings in appearance, quality, and comfort. The zero VAT saving means that a Grannexe one-bedroom annexe at £110,000 costs £110,000. A comparable brick build at £110,000 plus 20 percent VAT costs £132,000. That £22,000 difference could pay for premium interior finishes, a higher-spec kitchen, or simply remain in your bank account.

Brick and many timber frame builds: 20% VAT applies
Caravan Act compliant builds (including all Grannexe annexes): 0% VAT
On a £120,000 build, VAT saving is £24,000
Zero VAT is legal, HMRC-compliant, and well-established
Caravan Act requirements: max 20m x 6.8m, transportable in two sections
All Grannexe standard floor plans comply with Caravan Act dimensions

Interior Finishes and Specification

The interior specification has a significant impact on the final cost. A standard specification with laminate flooring, a functional kitchen with integrated appliances, a tiled bathroom, and painted walls is included in the base price from most quality builders including Grannexe. Upgrading to premium finishes can add £5,000 to £20,000 depending on what you choose. Common upgrades include engineered hardwood flooring (£2,000 to £4,000 extra), a premium kitchen with stone worktops and higher-end appliances (£3,000 to £8,000 extra), a luxury bathroom with walk-in rainfall shower and heated towel rail (£2,000 to £5,000 extra), and smart home features like automated heating control and integrated speakers (£1,000 to £3,000 extra). At Grannexe, our standard specification is already comprehensive. It includes a full kitchen with integrated appliances (fridge freezer, oven, hob, extractor, and space for a washing machine), a tiled bathroom with walk-in shower, underfloor heating throughout, LED lighting, and quality flooring. Most clients find the standard spec meets their needs without additional spending.

Standard specification: included in base price at Grannexe
Premium kitchen upgrade: £3,000 to £8,000
Luxury bathroom upgrade: £2,000 to £5,000
Engineered hardwood flooring: £2,000 to £4,000
Smart home features: £1,000 to £3,000
Total premium upgrades typically: £5,000 to £20,000
Standard Grannexe spec includes full kitchen, tiled bathroom, underfloor heating

Running Costs: What to Budget Monthly

The running costs of a granny annexe are often surprisingly low, especially in a well-insulated modern build. The main ongoing costs are electricity (including heating), water, and contents insurance. For a Grannexe annexe with WarmWall insulation and electric underfloor heating, typical monthly costs break down as follows. Electricity for heating, lighting, and appliances: £40 to £80 per month averaged across the year, with higher costs in winter and lower in summer. Water rates: £15 to £30 per month depending on usage and local rates. Contents insurance: £10 to £20 per month. Buildings insurance is typically covered under the main property's policy. The total monthly running cost is typically £65 to £130, or roughly £800 to £1,500 per year. This is remarkably affordable compared to any other form of independent accommodation. A one-bedroom flat in Kent or London would cost £800 to £1,500 per month in rent alone, before utilities. A care home costs £1,000 or more per week. The low running costs are a direct result of good insulation. Buildings that are poorly insulated (as many budget annexes and garden rooms are) can cost two to three times as much to heat.

Electricity (heating, lights, appliances): £40 to £80 per month
Water: £15 to £30 per month
Contents insurance: £10 to £20 per month
Total running costs: £65 to £130 per month
Annual running costs: approximately £800 to £1,500
Well-insulated annexes cost significantly less to heat
Compare to: rental flat £800 to £1,500/month, care home £4,000+/month

Hidden Costs to Watch Out For

The granny annexe market has its share of misleading quotes. Some builders advertise headline prices that exclude essential costs, making their product appear cheaper than it actually is. Here are the most common hidden costs to ask about before signing any contract. Site preparation is frequently excluded from budget quotes. If the quote says 'price excludes groundwork,' find out what groundwork actually costs before committing. Utility connections are another area where costs can be hidden. Some builders quote for the annexe only, leaving you to arrange and pay for electricity, water, and drainage connections separately. Planning permission fees and application management are sometimes excluded. The planning application fee itself is modest (around £230 for a householder application), but the drawings, design statements, and project management involved can cost £1,000 to £3,000 if you need to hire a separate planning consultant. Delivery and crane hire are costs specific to modular buildings. The annexe panels need to be transported to your property and lifted into position. Some builders include this; others do not. Internal finishes can also inflate the price. A quote that includes 'shell only' or 'weather-tight shell' means you still need to pay for the kitchen, bathroom, flooring, decorating, and all internal fit-out.

Site preparation and groundwork
Utility connections (electricity, water, drainage)
Planning permission application and drawings
Delivery and crane hire for modular builds
Internal fit-out if only a shell is quoted
Landscaping and external paths
Building regulations fees (for non-Caravan Act builds)
At Grannexe, our quotes are fully inclusive with no hidden extras

How a Granny Annexe Affects Your Property Value

A well-built granny annexe is one of the best investments you can make in your property. Research consistently shows that homes with a self-contained annexe sell for 20 to 30 percent more than comparable properties without one. On a property worth £400,000, that is an uplift of £80,000 to £120,000, significantly more than the cost of building the annexe in the first place. The demand for homes with annexes has grown substantially in recent years, driven by the rising cost of care, multi-generational living trends, and the flexibility that an annexe provides. An annexe can serve as accommodation for elderly parents, returning adult children, a home office, or rental income (subject to planning). Estate agents report that properties with annexes attract more viewings and sell faster than those without. However, this value uplift depends on the quality of the annexe. A poorly built garden room being marketed as a granny annexe will not add the same value as a properly constructed, well-insulated, self-contained home. Build quality matters for resale just as it matters for daily living.

Typical property value increase: 20 to 30 percent
On a £400,000 property: £80,000 to £120,000 uplift
Properties with annexes sell faster and attract more viewings
Demand driven by care costs, multi-generational living, and flexibility
Quality of the annexe directly affects the value uplift
Grannexe builds are designed to maximise long-term property value

Conclusion

The cost to build a granny annexe in 2026 ranges from £30,000 for a basic garage conversion to over £200,000 for a luxury two-bedroom brick build. For most families, the sweet spot is a quality modular annexe in the £90,000 to £150,000 range, built to Caravan Act standards with zero VAT. At Grannexe, our annexes start from £90,000, include our WarmWall insulation system, full kitchen and bathroom, and a 10-year structural guarantee. We have been building across Kent, London, and the South East for over 20 years, and our fully inclusive pricing means no hidden costs. If you want an accurate figure for your specific situation, the best next step is a free site survey and consultation. We will assess your garden, discuss your requirements, and provide a detailed quote with no obligation.

Frequently Asked Questions

How much does it cost to convert a double garage to a granny flat in the UK?

The cost to convert a double garage to a granny flat in the UK ranges from £30,000 for a basic conversion to £70,000 for a high-end finish. Mid-range conversions with proper insulation, a good kitchen, and a tiled bathroom typically cost £40,000 to £55,000. Remember to budget for building regulations approval, planning permission if required, and any structural modifications.

How much does a one-bedroom granny annexe cost?

A purpose-built one-bedroom granny annexe costs between £100,000 and £160,000 depending on the builder, specification, and construction method. At Grannexe, one-bedroom annexes start from £110,000 with zero VAT under the Caravan Act. This includes the full build, kitchen, bathroom, heating, insulation, and utility connections.

Do you pay VAT on a granny annexe?

It depends on the construction method. Brick-built annexes attract 20% VAT. Annexes built under the Caravan Act 1960, including all Grannexe builds, are zero-rated for VAT. On a £120,000 build, the difference is £24,000. This is completely legal and HMRC-compliant.

What are the running costs of a granny annexe?

A well-insulated modular granny annexe typically costs £65 to £130 per month for all utilities (electricity, heating, water) plus contents insurance. Annual running costs are approximately £800 to £1,500. Poorly insulated buildings can cost two to three times as much to heat.

Is it cheaper to convert a garage or build a new granny annexe?

A garage conversion is cheaper in absolute terms (£30,000 to £70,000 vs £90,000 to £150,000 for a purpose-built annexe). However, a purpose-built annexe offers better insulation, optimal room layout, zero VAT potential, and typically adds more value to your property. The long-term value of a purpose-built annexe usually justifies the higher upfront cost.

How long does it take to build a granny annexe?

A modular granny annexe from Grannexe takes 12 to 18 weeks from contract to handover. A traditional brick build takes 4 to 6 months. A garage conversion typically takes 8 to 12 weeks. Add 8 to 12 weeks for planning permission if a formal application is required.