Garage Conversion Problems: Signs, Causes and Fixes

Why do garage conversions go wrong?

A garage is built to keep a car dry, not to be lived in. Its walls are often a single skin of brick, the floor is bare concrete set lower than the house, the roof may be thin felt or sheeting, and nothing about it is designed to hold heat. A good conversion deals with all of that. A poor one covers it up: plasterboard straight onto cold brick, carpet onto an unprotected slab, and a few blocks laid across the old door opening.

The trouble is that many of these shortcuts look fine on day one. Problems show up over the first winter or two, as condensation builds behind furniture, damp rises along the skirting, and the infill wall settles. By then the room is decorated and in use. Many of the calls we get when converting garages across Merseyside are about conversions done years ago, sometimes by a previous owner, that never had the detailing right.

What signs should you look for?

Whether you are living with a converted garage, buying a house with one, or checking work in progress, these are the things worth noticing:

• Tide marks at skirting levelDamp staining, bubbling paint or swollen skirting along the bottom of the walls, often worst on the outside walls.
• Black mould in cornersMould behind wardrobes, in the corners where walls meet the ceiling, or around the window.
• A room that never feels warmCold walls to the touch, a cold floor, or a room that loses heat soon after the heating goes off.
• A step downThe floor sits noticeably lower than the hall or kitchen, sometimes with a ramp or single step.
• Cracks around the old openingVertical or stepped cracks where the infill wall meets the original brickwork, inside or out.
• Musty smellA damp, stale smell, especially after the room has been closed up.
• Tripping electricsA circuit that trips, warm sockets, extension leads everywhere, or wiring that looks like it came from a garage supply.
• No way outA bedroom reached only through another room with no opening window large enough to escape through.

Sign, likely cause and what to do

Damp at the base of the walls

This is the problem we see most. When a garage floor is converted, a damp-proof membrane goes down before the insulation and new floor. LABC guidance says that membrane has to lap with the damp-proof course in the existing walls, so there is no gap for ground moisture to rise through. In a lot of older conversions, the membrane stops short, or there was never a membrane at all, and the new floor was laid straight onto the old slab. Moisture then wicks up through the junction and shows as tide marks at skirting level.

It can also come from outside. If a patio, path or flower bed has been built up against the old garage wall above the damp-proof course, rain splashes and soil moisture can bridge it. Checking ground levels outside is a sensible first step, because it is easy to see and sometimes simple to put right.

Fixing the floor junction properly usually means lifting the floor along the affected walls, or the whole floor, and rebuilding it with the membrane correctly lapped. Our page on garage floor insulation and levelling explains how a new floor is put together.

Cold walls, condensation and mould

Many garage walls are half-brick: a single skin of brick about 100 mm thick. Rain soaks into it and it loses heat quickly. If a conversion simply plasterboards over that brick, the inside surface stays cold, warm moist air from the room condenses on it, and mould follows, particularly behind wardrobes, beds and sofas where air cannot move.

LABC guidance says single-leaf garage walls need treatment to make them weatherproof, often by building a new insulated inner leaf, or a framed inner wall with a cavity and membrane. Where piers have been removed from a single-skin wall, the wall is weaker and should be tied to the new inner leaf with remedial wall ties. Opening windows and running an extractor helps in the short term, but it will not fix a wall that is simply too cold. See garage wall insulation for the ways this is done.

A step down into the room

Garage floors are often set lower than the house. On integral garages this was frequently deliberate, so that fuel or spills could not run into the house. A conversion normally builds the floor up with membrane, insulation and screed or timber, which raises the level and removes or reduces the step.

When a conversion keeps the old level, you get a step that people trip on, a cold floor, and usually no insulation underneath. It is not just an inconvenience: for older relatives or anyone with limited mobility, a step in a doorway is a real hazard. Rebuilding the floor is disruptive, but it fixes the cold, the damp and the step at the same time.

Cracks in the infill wall

Where the garage door used to be, there is often no foundation under the opening, because the door simply sat on the slab. The Planning Portal notes that a new foundation may be needed for the new wall, and LABC says the wall can sometimes be carried on a suitable lintel or beam instead, with the method agreed with building control.

When a previous builder laid blocks straight onto the slab, the wall can settle differently from the original brickwork around it, and cracks appear at the joints. Fine hairline cracks may be shrinkage; cracks that widen, step through the brickwork, or let daylight or rain through need a proper structural look. The fix depends on what is underneath, so it has to be investigated before anyone can say what is needed. Our page on garage door infill explains the options for a new infill.

A bedroom with no safe way out

If a converted garage is used as a bedroom and you can only reach it by going through another room, such as a kitchen or living room, it is what building control calls an inner room. LABC guidance asks for an escape window in that situation, with a clear opening of at least 450 mm by 733 mm and the bottom of the opening no more than 1.1 m above the floor. It also asks for smoke alarm cover in the area outside the converted room.

Older conversions sometimes have a small fixed window, or a window that opens but not far enough to climb through. If you are using a converted garage as a bedroom and are unsure, treat it seriously. Building control can tell you what applies. Our page on garage to bedroom conversions covers what a bedroom needs.

A related problem in integral garages is fire separation. Before conversion, the wall and door between an integral garage and the house usually provide fire separation, and the internal door is normally a fire door. After conversion the fire strategy for the whole floor should be reviewed with building control. Fire doors should not simply be removed.

Unsafe or overloaded electrics

A garage often has a single circuit meant for a light and a socket. When that is extended to power a bedroom, office or gym with heaters and computers, it can be overloaded. In some older conversions the wiring was done without notification and without an Electrical Installation Certificate.

Warning signs include breakers that trip, sockets or plugs that feel warm, scorch marks, flickering lights, and heavy use of extension leads. Get a registered electrician to inspect the installation. Notifiable electrical work in a conversion falls under Part P, and should be done by an electrician registered with a competent person scheme, who issues a certificate and notifies building control. Our page on electrics for garage conversions explains the paperwork.

Roof leaks and asbestos

Attached and detached garages often have flat felt roofs, which have a limited life, or single-skin pitched roofs with no insulation. If the roof was not upgraded during the conversion, you can get leaks, ceiling stains, and condensation in a cold roof void.

Older garages sometimes have corrugated or flat cement sheets on the roof, which may contain asbestos. These must be tested by a qualified surveyor and, if needed, removed by a suitably qualified asbestos contractor before any work. We never disturb suspected asbestos, and you should not drill, cut or break it either. See garage roof upgrades for more on the options.

No completion certificate

This one often only comes to light when a house is being sold. A buyer’s solicitor will usually ask for the building control completion certificate for a garage conversion, and planning paperwork if it was needed. If there is none, the sale can be delayed.

The options are usually regularisation, a retrospective application to the council’s building control, or indemnity insurance. Regularisation can involve opening up parts of the work so an inspector can see the floor, walls or foundations, and may lead to remedial work. Which route is right depends on the property and the sale, so get advice from your solicitor. Our page on Building Regulations for garage conversions explains how approval normally works.

Planning problems after the event

Some houses, particularly on newer estates and in conservation areas, have had their permitted development rights removed, often by a condition on the original planning permission or an Article 4 direction. If a conversion was done without checking, it may be in breach of a planning condition. Detached garages used as separate living space can also raise planning issues. If you are unsure, check your property’s planning history with your council. Our page on planning for garage conversions explains what to look for.

How can you avoid these problems in a new conversion?

• Get the garage looked at properly before any price is agreed: walls, floor level, roof, door opening and services.
• Check planning history with your council before work starts.
• Make sure the job has Building Regulations approval, with inspections at the usual stages.
• Ask how the floor membrane will link to the wall DPC, and how the walls will be insulated and kept dry.
• Ask how the infill wall will be supported, and confirm building control has agreed it.
• If it will be a bedroom, ask about the escape window and smoke alarms.
• Make sure electrics and gas work are done by suitably registered trades, with certificates.
• Keep the completion certificate and electrical paperwork with your deeds.

The cost of doing these things properly is part of what is explained in our garage conversion cost guide, and it is far less than putting them right later.

When should you call a professional?

Call for advice if damp keeps coming back after redecorating, if mould returns after cleaning, if cracks are growing, if circuits trip, if a bedroom has no proper escape route, or if you are selling and cannot find the paperwork. Also get help before any work to walls, floors, roofs or electrics, and if a roof might contain asbestos. We look at problem conversions across Liverpool and Merseyside, from Croxteth Park to Formby, and we can explain what is causing the problem and what it would take to fix, whether that is a repair or a full garage conversion done properly from the start.