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Converting an outbuilding into guest accommodation

Michael Holmes
Renovation expert Michael Holmes

Turning a summerhouse, detached garage or another outbuilding into a self-contained annexe is an ideal way of providing private accommodation for guests, elderly parents, or twentysomething offspring who are still living at home but need some independence. Michael Holmes offers his advice on how to go about doing this.

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Do I need planning permission?

Converting an outbuilding in your garden into a residential annexe doesn’t require planning permission as it constitutes permitted development, provided that the use of the annexe is ‘incidental to the enjoyment of the dwelling house’.

This means the annexe can be used as a home as long as it is occupied only by guests, members or dependents of the main household. It cannot be let to anyone else without first getting planning permission for ‘independent use’. Planners are often reluctant to grant permission for independent use, as the building could later be sold off separately from the main house and potentially be replaced by a new, larger dwelling, allowing almost infinite sub-division of gardens into plots.

Anyone looking to get planning permission to build a new annexe would do well to write down a different use on the application form, complete the building and then change the use to an annexe at a later date.

Planning permission may not always be necessary, as there is considerable scope to build new outbuildings covering up to 50 per cent of a house’s original garden, up to four metres in height with a pitched roof, or three metres with a flat roof, using permitted development rights.
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Do building regulations apply?

If the outbuilding is to be used as a habitable room then it must be upgraded to meet the Building Regulations requirements for ‘a material change of use’. The structure will need to be insulated, while doors and windows are likely to require double glazing. Any new heating, electrics and plumbing will also need to comply. As the work is notifiable under Building Regulations, you must inform the local authority, complete the relevant forms and pay a fee.
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Can I connect to the mains?

It is usually straightforward to connect an outbuilding to the mains electricity and water supplies, telephone and mains sewers, and can be done without the help of the utility companies.

The annexe can have its own heating system, either a small gas or oil-fired combi boiler, or a small electric water heater, depending on demand. A gas supply cannot be altered other than by someone on the Gas Safe Register (
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Will there be a council tax bill?

Although a separate self-contained annexe can be useful, it does have a drawback in that it will be registered separately for council tax. The tax is typically levied at the lowest band (depending on the size and value of the annexe) and is payable even when the building is empty – although there is a 50 per cent reduction for the first six months after it becomes unoccupied.

The only way to have an annexe disaggregated to lift the council tax liability is to fully remove either the kitchen or the bathroom and all the associated plumbing.
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Hi Michael,

I'm not sure when you wrote this article and therefore which planning regs apply but I recently saw 'George Clarkes Amazing Spaces' in which he said that the height restriction was 2.5 metres (he didn't specify a difference with a pitched roof). Is it, as you say above 4m & 3m or 2.5m?

many thanks,