Converting an outbuilding into a usable room

Why convert rather than build new

Plenty of gardens already have a brick outbuilding: a detached garage, an old wash house, a workshop or a stable. Converting it keeps the footprint you already have, often avoids a fresh planning debate about a new structure, and reuses walls that may be perfectly sound. The catch is that these buildings were never designed to be warm or dry inside, so the work is mostly about bringing an old shell up to a living standard.

Start with the structure

Before planning the interior, check the bones. Look for cracks in the walls, a sagging roof line, rotten timber at wall plates and rafters, and whether the building has any real foundation. Many older outbuildings sit on shallow brick footings or a thin slab. That can be fine for a store but not for a heavier roof or a new floor build-up.

Walls are often single brick, which is thin, cold and prone to damp. They may need an inner leaf of blockwork or an insulated lining, and any cracked or loose brickwork should be repaired first.

Floors, damp and insulation

The existing floor is usually a bare concrete slab with no damp-proof membrane. A typical upgrade is a membrane, rigid insulation and a screed or timber floor on top, which raises the level, so check door heights and headroom before you commit. Walls need a damp-proof course and insulation, and the roof usually needs either insulation between and under the rafters or a new warm roof.

Condensation is the problem people underestimate. A sealed, insulated room with no ventilation will run with water in winter, so trickle vents and an extractor are part of the plan, not an extra.

Windows, doors and services

Old garage doors and single-glazed windows are replaced with insulated walls or new glazed doors. Electrics need a proper supply run from the house, usually buried in a duct. Water and waste are the big decision: a toilet or kitchenette needs a connection to the drains, and if the outbuilding is far from the house or uphill of the sewer that can be a significant part of the job.

Planning and Building Regulations

Using an outbuilding for something incidental to the house, such as a home office, gym or hobby room, often needs no planning consent. Turning it into self-contained living accommodation with a bedroom and kitchen usually does. Listed buildings and conservation areas add their own rules.

Building Regulations apply once you are creating a habitable room, covering structure, insulation, fire escape, ventilation and electrics. Getting Building Control involved early saves arguments later.

Our conversion work

Outbuilding and annex conversions are one of the services we list under conversions. Our own case study of a garage extended into a separate music practice room shows the sort of result possible when an outbuilding is properly insulated and finished. We are a Dunstable-based team with more than 26 years' experience, NVQ Level 2 brickwork, FMB membership and full insurance.

If you are thinking of a brand-new garden building instead, the garden room guide covers that route.

A sensible order of work

Most successful outbuilding conversions follow the same sequence. First, a survey of the walls, roof and floor to decide what can stay. Second, confirm the intended use and check planning and Building Regulations before spending money on design. Third, carry out structural repairs and any new foundations or wall linings. Fourth, make the building weathertight with the roof, windows and doors. Only then do the floor build-up, insulation, services and finishes go in.

Doing it in this order avoids the classic mistake of fitting a nice new floor or plastered walls inside a shell that still leaks or moves. It also lets Building Control inspect each stage while the work is still visible, rather than asking for things to be opened up later.

Budget a contingency for what you find once old linings, floors and roof coverings come off. On older buildings there is almost always something.

Questions people ask