How Much Does a Granny Annexe Cost?
Last updated: April 2026 · Based on 176 published prices from UK annexe specialists
How Much Does a Granny Annexe Cost in the UK?
Advertised starting prices for granny annexes in the UK range from around £52,000 for a compact studio to £155,000 or more for a large two-bedroom building. Most standard one-bedroom annexes — typically 35–50sqm — are priced between £70,000 and £115,000.
These are advertised "from" prices, not average installed costs. The final figure depends on size, specification, which planning route is used, and what is included in the quote. Groundworks and mains connections are frequently excluded from advertised prices and can add a further £15,000–£20,000 to the total.
Use the calculator below to see typical published price ranges by size.
Granny Annexe Market Price Calculator — based on April 2026 survey data
Step 1 — Choose a planning route
Step 2 — Select a floor area
↑ Choose a floor area above to see typical starting prices
- These are advertised starting prices — real quotes will vary based on your specification, site conditions, and supplier. Always get at least three quotes.
- VAT inclusion in price quoted varies — companies don’t always make clear whether VAT is included. Note: mobile homes built to BS3632 standard may qualify for zero-rated VAT in some cases, which can make the Caravan Act route more cost-effective than it first appears. Always confirm VAT status with your supplier and accountant.
- Groundworks usually priced separately — foundations and site preparation costs vary depending on your site. Always confirm what is included when requesting a quote.
- Caravan Act vs Full Planning — Caravan Act (mobile home) classification avoids full planning permission in most cases. Full Planning gives a permanent, Building Regulation-certified structure. Both routes have different long-term implications — speak to a specialist.
Find an annexe specialist near you
Not all companies work in every area — use the directory to find those covering your location.
Garden Room & Annexe Directory Browse Caravan Act annexe specialists working in your area →Our price data is drawn from 176 published prices collected from UK annexe specialists in April 2026. All are advertised starting prices — upgrades, site-specific costs, and optional extras are not reflected. The number of published prices examined for each size band is shown alongside each figure in the calculator.
Why Annexe Prices Are Difficult to Compare
Comparing annexe prices is harder than it looks. Three factors create most of the confusion.
VAT is not consistently stated
Some companies include 20% VAT in their advertised prices; others do not state it at all. At 20%, a £95,000 price becomes £114,000 inclusive. Companies that include VAT can appear more expensive at first glance — or the difference is real and reflects a genuinely higher price. Always confirm VAT status before making any comparison.
Groundworks are often excluded
Foundation installation, site preparation, and connections to mains electricity, water, and drainage are the single biggest wildcard in annexe pricing. Some companies include them; many do not. Where both building and fully-installed prices were available in our survey, site preparation and connections typically added £15,000–£20,000 to the building cost. A quote that looks cheaper on paper may simply have fewer items in it.
All advertised prices are "from" prices
Kitchen specification, bathroom fittings, cladding choices, window configuration, accessibility adaptations, and Lawful Development Certificate applications all affect the final figure. The published starting price is always a floor, not a ceiling.
Caravan Act or Full Planning
The planning route is the most significant structural decision in any annexe project. It affects the process, the price, and potentially the VAT position.
The Caravan Act route
Most UK annexe specialists build to the BS3632 residential standard, which allows the building to be classified as a mobile home under the Caravan Act. No full planning application is required in most circumstances. Instead, a Lawful Development Certificate confirms the development is lawful — some companies include this in their price, others pass the cost on as an extra.
Ground screw pile foundations are often used on this route, since the building must remain technically moveable to meet the classification. The process is faster and simpler than full planning.
There is also a potential VAT advantage. Mobile homes built to the BS3632 standard may qualify for zero-rated VAT under UK VAT rules, rather than the standard 20%. Always confirm VAT treatment directly with the supplier and an accountant.
Full Planning Permission and Building Regulations
Some companies build conventionally-planned annexes requiring a full planning application and Building Regulations sign-off. This route takes longer and may cost more, but the result is a permanent structure treated as an extension in legal and mortgage terms. Planning permission is granted by the local authority — approval is not guaranteed, and the process can take several months.
Neither route is inherently better. The right choice depends on the property, the local authority, and the buyer's long-term intentions for the building.
What Affects the Cost of a Granny Annexe?
Size
Annexes are broadly priced per unit rather than strictly per square metre, but larger buildings cost more. A compact studio of around 20sqm can start from approximately £52,000 on the Caravan Act route; a large two-bedroom annexe of 75–80sqm can reach £155,000 or more.
Size and price do not scale proportionally. Most of the cost is fixed — kitchen, bathroom, utilities connections, delivery, and installation — regardless of floor area. A 20sqm studio typically costs around 60–70% of the price of a 40sqm one-bedroom annexe, not 50%. Buyers working to a firm budget are often better served by pricing the next size up before concluding it is out of reach.
Specification and finish
Base prices cover the structure and a standard specification. Kitchen fitout, bathroom fittings, flooring, cladding choices, and window configuration all affect the final figure. Some companies offer fixed packages; others quote bespoke. Published starting prices are always a starting point — a fully-specified annexe with upgraded finishes will cost more.
Groundworks and site preparation
Foundations, site preparation, and connections to mains services are frequently excluded from advertised prices. Ground conditions, access, the distance to existing mains connections, and how level the site is all affect the cost. Always ask what groundworks allowance is built into a quote, and whether the company carries out groundworks directly or subcontracts.
VAT
VAT treatment varies across the market. Some companies include 20% VAT in their advertised prices; others quote ex-VAT or do not state it. Caravan Act structures may qualify for zero-rating in some circumstances. Confirm VAT status and rate before comparing any two quotes.
What's Included in an Annexe Price?
Advertised starting prices typically include:
- The structural frame and insulation
- External cladding and roofing to a standard finish
- Windows and external doors
- Internal studwork and plasterboard
- A standard kitchen and bathroom fitout
- First-fix electrical work — consumer unit, sockets, lighting positions
- Delivery to site
They typically exclude:
- Groundworks and foundations (unless explicitly stated)
- Connection to mains services — electricity, water, and drainage
- Planning application or Lawful Development Certificate fees
- Furniture, white goods, and accessories
The gap between building price and installed cost
Where comparable building and fully-installed prices were available in our survey, site preparation, foundation installation, and mains connections added £15,000–£20,000 to the building price. This figure held broadly consistent regardless of building size. It is the most reliable market evidence available of what typically sits between an advertised starting price and a finished, connected annexe ready for occupation.
Some companies offer a fully turnkey price covering groundworks and connections. Always ask — and make sure any quotes you are comparing cover the same scope of work.
Frequently Asked Questions
How much does a granny annexe cost in the UK?
Advertised starting prices range from around £52,000 for a compact studio to £155,000 or more for a large two-bedroom annexe. Most standard one-bedroom annexes (35–50sqm) are priced between £70,000 and £115,000. These are starting prices — groundworks and mains connections typically add a further £15,000–£20,000.
What is the Caravan Act route for annexes?
The Caravan Act route allows annexes built to the BS3632 residential mobile home standard to be classified as mobile homes. No full planning application is required in most circumstances. A Lawful Development Certificate confirms the development is lawful. Ground screw pile foundations are often used so the building remains technically moveable. There may also be a VAT advantage — some supplies on this route qualify for zero-rated VAT.
What is the difference between a Caravan Act annexe and a Full Planning annexe?
Caravan Act annexes are classified as mobile homes, require no full planning application, and can typically be installed more quickly. Full Planning annexes are permanent structures requiring a planning application and Building Regulations approval — a longer process, but the result is legally equivalent to a conventional extension.
Do I pay VAT on a granny annexe?
It depends on the route and how the supply is structured. Standard construction attracts 20% VAT. Annexes built to the BS3632 standard under the Caravan Act may qualify for zero-rated VAT in some cases — but this is not universally applied. Confirm VAT status and rate directly with the supplier and an accountant before comparing quotes.
What is not included in an annexe price?
Advertised prices frequently exclude groundworks and foundations, connection to mains services, planning or Lawful Development Certificate fees, and internal decoration. Site preparation and mains connections typically add £15,000–£20,000 to the building price.
How big does a granny annexe need to be?
A compact studio of 20sqm can work for occasional guest use. A standard one-bedroom annexe of 35–50sqm is more suited to full-time occupation, providing a comfortable bedroom, bathroom, kitchen, and living area. Two-bedroom annexes of 65–80sqm suit couples or where additional flexibility is needed.
Getting Quotes
Allow time to gather at least three quotes. When approaching companies, ask each one:
- Does this price include VAT? At what rate?
- Are groundworks and foundations included?
- Are mains connections — electricity, water, and drainage — included?
- Which planning route are you proposing, and why is it right for this site?
- What does the Lawful Development Certificate process involve, and who manages it?
The answers will quickly show whether two quotes are genuinely comparable — or whether one includes items the other has left out entirely.
