Two-bedroom granny annexes
A second bedroom turns an annexe from a home for one into a home for a couple, or for a parent with a carer who stays overnight. It also adds roughly a third to the floor area, and with it to the cost.
Two bedrooms, about 45 m²
£99,000 to £144,000
Including VAT, standard foundations included. A guide, not a quote.
Who needs the second room
Three situations come up again and again. A couple who sleep apart, often because one is unwell or restless at night. A parent who needs a carer to stay some or all nights. And a parent who simply wants a guest room, so grandchildren or a sibling can stay.
If the second room is for a carer, plan it as a proper bedroom with a window, a door that closes and space for a wardrobe. A box room will not keep a good carer for long.
How much floor it takes
Most two-bedroom annexes are between 40 and 65 m² inside. Below about 40 m², one of the bedrooms or the bathroom ends up too small to be comfortable.
| Floor area | What you can expect | Guide cost with VAT |
|---|---|---|
| 40 m² | Two modest bedrooms, one shower room, kitchen in the living room | £88,000 to £128,000 |
| 45 m² | One double and one single or twin, a wet room, a room to sit in and eat | £99,000 to £144,000 |
| 55 m² | Two doubles, a larger bathroom, space for a dining table | £121,000 to £176,000 |
| 65 m² | Two doubles, a bathroom and a second WC, a separate kitchen | £143,000 to £208,000 |
Figures cover the full range of finishes, from plain to high specification, and include VAT. Most annexes cost under £100,000; a two-bedroom plan is where budgets most often pass it.
Layouts that work
The plan most families end up with puts the living room and kitchen in the middle, facing the garden, with a bedroom at each end. Each bedroom gets some privacy from the other, and both are close to the bathroom.
If one bedroom is for a carer, give the main bedroom the shortest, clearest route to the bathroom, ideally a door straight into it. Put the carer's room nearest the front door, so they can come and go at night without waking anyone.
For a wheelchair user, allow a turning circle of about 1.5 m in the main bedroom, the bathroom and the kitchen. It is the single biggest reason two-bedroom annexes end up larger than first sketched.
Planning a larger annexe
The bigger an annexe, the harder a council looks at whether it is really part of the main house or a separate home in all but name. A two-bedroom annexe is often approved, but expect the planning officer to ask who will live there and how the household will work. A short, honest statement with the application helps.
The twin-unit caravan route can be used for a two-bedroom layout: the legal limits are 20 m long and 6.8 m wide, with 3.05 m inside height, in no more than two sections. Read our planning guide before relying on it.
Keeping the cost down
Floor area is where the money goes. Two bedrooms with a shared shower room cost less than one bedroom with an en suite plus a second bathroom. A simple rectangle with a single roof pitch is cheaper to build than an L shape. And fitting the kitchen along one wall of the living room saves both floor and a wall.
Try different sizes and finishes in the cost calculator to see where your budget lands.
Where our information comes from
Build prices are our own national guide ranges for 2026 for a finished annexe, drawn from published UK cost guides and shown including VAT. London and the South East usually cost 20 to 40% more. They are a guide, not a quote. Rules on planning, tax and building control come from the official sources below and are summarised for England unless we say otherwise.
- GOV.UK: permitted development rights for householders, technical guidance
- The Town and Country Planning (General Permitted Development) (England) Order 2015, Class E
- Caravan Sites Act 1968, section 13 (twin-unit caravans)
- The Caravan Sites Act 1968 (Amendment of Section 13(2)) (England) Order 2006
- Approved Document M: access to and use of buildings
- GOV.UK: building regulations approval
- HMRC: VAT Notice 708, buildings and construction
Two-bedroom annexe questions
How much does a 2 bedroom granny annexe cost?
As a guide, £99,000 to £144,000 including VAT for about 45 m², finished. Smaller plans start lower and larger ones with two double bedrooms run higher.
Is a 2 bedroom annexe harder to get planning for?
It can be. Councils check that a larger annexe will stay part of the main household. Explain who will live in it and how, and expect a condition that it cannot be sold or let separately.
Can a 2 bedroom annexe be a mobile home?
Yes, if it stays within the legal size of a twin-unit caravan: 20 m by 6.8 m, 3.05 m inside height, in no more than two sections.