Painting a Garage Conversion: From Concrete Floor to Habitable Room

Garage Conversions: A Different Set of Challenges

Converting a garage to habitable space has become one of the most common home improvement projects in London over the past decade. Integral garages in Belgravia, Pimlico, and Kensington mews properties are being converted into studies, home gyms, utility rooms, and extra bedrooms with increasing frequency. From a decorating perspective, these spaces present a distinct set of technical challenges that differ from redecorating an existing room.

The issues are: a concrete slab floor that must be treated before any floorcovering or painted finish; walls that are typically newly constructed with insulated plasterboard (PIR-backed plasterboard) and require a specific approach to decoration; and the requirement to understand the damp proof membrane that should be part of any properly executed conversion.

The Concrete Floor

Almost every garage has a concrete slab floor, and that floor must be properly addressed before the conversion is complete. There are two common scenarios.

Scenario 1: Painted concrete floor. Where the client wants to retain a painted concrete floor — common in utility rooms, gyms, and boot rooms — the correct approach is to apply a specialist concrete floor paint rather than a standard emulsion. The options:

Rustins Concrete Floor Paint — a solvent-based, single-component floor paint with good hardness and reasonable durability. Suitable for lighter use.

Dulux Trade Heavy Duty Floor Paint — a water-based polyurethane floor paint designed for industrial and commercial use. More durable than single-component options, with better resistance to abrasion and mild chemical spillage (motor oil, cleaning fluids). The appropriate choice for a garage conversion that sees heavy foot traffic or wheeled equipment.

Remmers epoxy floor coating — for the most demanding applications, a two-part epoxy system provides superior hardness, chemical resistance, and longevity. More expensive and requires more careful application (pot life management, precise mixing ratios) but provides a genuinely durable result.

Before applying any floor coating, the concrete must be clean, dry (below 75% relative humidity measured with a surface hygrometer or 5% by weight), and free of any existing oil contamination. Degrease with a TSP (trisodium phosphate) solution or a specialist concrete degreaser. Allow to dry fully. Apply a concrete primer such as Rustins Concrete Sealer or Remmers EP Primer before the topcoat.

Scenario 2: Floor to receive a floorcovering. If the client is installing vinyl, engineered wood, or tiles over the concrete, the floor must still be level and sealed to prevent moisture migration upward. Self-levelling compound (Mapei Ultraplan, Ardex K15) applied to the concrete after any DPM treatment brings the floor to a flat, paintable surface that floorcoverings can be laid over directly.

Damp Proof Membrane: Understanding What Should Be There

A properly executed garage conversion will include a horizontal DPM — typically a 1200-gauge polythene sheet or a bitumen-based painted membrane — applied either below the new insulated floor build-up or to the existing concrete slab before screeding.

The DPM prevents moisture rising from the ground through the slab (rising damp) and humidifying the habitable space above it. Without a DPM, or where the DPM is inadequate or punctured, floor coverings will lift, timber studwork can rot at sole plate level, and condensation may form on cold surfaces.

As a decorator, your role is to identify if the DPM appears to have been correctly executed (visible at perimeter, turned up the walls and overlapping the cavity tray) and to flag any evidence of dampness — tide marks on the lower walls, a white crystalline efflorescence on the concrete floor, or a persistently cold and damp feel to the slab — before any decorative work begins. Painting over active damp is not a solution; it is a way of masking a problem that will re-emerge and require more expensive remediation later.

Insulated Plasterboard Walls

Garage conversions are commonly lined with PIR-backed plasterboard — a composite board with a rigid polyisocyanurate foam layer bonded to a standard plasterboard face. Brands include Celotex PL4000, Kingspan Kooltherm K118, and Recticel Eurothane. This material provides thermal insulation and a paintable surface in a single board.

The key differences from standard plasterboard that affect decoration:

The surface is more uniform but more absorbent. New PIR-backed plasterboard surfaces absorb the first coat of paint faster than standard plasterboard, making a proper mist coat (10–15% water dilution of the finish emulsion) essential before full-strength topcoats are applied.

Joints must be taped and filled. Butt joints between boards should be taped with joint tape and filled with Gyproc jointing compound or equivalent, feathered out at least 150mm either side of the joint and sanded smooth before any paint goes on. Un-taped joints will telegraph through the paint finish.

Check for a skim coat. If the builder has applied a thin skim of finish plaster over the boards (common in better-quality conversions), allow four weeks minimum for the plaster to dry before applying paint.

Boards must be dry. Moisture trapped in the board or the foam layer, whether from construction or subsequent damp ingress through an inadequate DPM, will cause paint to blister and peel.

Product Choices for Habitable Use

Once the walls are prepared and primed, the product choices for a converted garage follow broadly normal interior principles, with a few specific considerations:

Moisture-resistant emulsion is worth specifying even in a conversion that appears dry — particularly for ground-level rooms in London where below-ground humidity is a persistent issue. Little Greene Intelligent Matt has better moisture resistance than standard emulsions. Zinsser Perma-White is a mould-resistant formulation useful in utility rooms and boot rooms that will see wet clothing and footwear.

Eggshell on woodwork — door frames, skirtings, and any timber elements should be finished in a durable water-based eggshell. The conversion environment, with its potential for greater humidity variation than an upstairs room, rewards harder finishes.

Avoid dead-flat emulsions on lower walls — particularly below 600mm from the floor in a converted garage. This zone is most exposed to moisture, cleaning, and general contact. A wipeable finish in this zone is practical.

Making the Space Feel Like a Room, Not a Garage

Colour choice matters significantly in a conversion. Ground-floor or below-grade rooms benefit from warm colour palettes — Farrow & Ball Elephant's Breath, Little Greene Aged Paper, Dulux Magnolia Warm White — rather than cool greys or whites that amplify any feeling of dampness or institutional plainness. Adequate artificial lighting, planned before decoration rather than added as an afterthought, is equally important.

Ready to Decorate Your Conversion?

We work on garage and basement conversions across South West London and the wider SW1 area. If your conversion is nearing completion and you need a professional decorator to bring it to a habitable standard, contact us for a free quote.