Painting a New House Extension in London: New Plaster, MDF, Plasterboard Sequences and Getting the Finish Right
A technical guide to decorating new house extensions in London. Covers new plaster and plasterboard sequences, MDF and joinery preparation, matching existing rooms, and avoiding the mistakes that cause early failure.
The Extension Is Built. Now What?
A new rear or side extension in London typically completes after four to eight months of construction. By the time the scaffolding comes down and the kitchen is fitted, most clients want the decoration done quickly so the house can return to normal. This pressure to finish fast is the single most common cause of premature paint failure in newly decorated extensions.
The issues are not complicated, but they are unforgiving if ignored. New building work introduces moisture — from concrete, from fresh plaster, from timber framing — that must be allowed to dry out before any decoration is applied. It also introduces surfaces that have not been decorated before: plasterboard, MDF joinery, and metal window frames, each of which needs a different preparation sequence from the old plaster walls in the rest of the house.
New Plaster: The Non-Negotiable Drying Period
A standard 3mm skim coat on a new extension needs a minimum of four weeks to dry in normal conditions. In a north-facing extension in winter, or in a room without heating or ventilation, this period extends to six weeks or more. The test is colour: new plaster dries from a dark, patchy grey-pink to a consistent pale pink-white. If any areas are still darker than the surrounding plaster, the moisture content is still too high.
Painting wet or incompletely cured plaster causes the moisture to migrate through the paint film as the plaster continues to dry. The result is blistering and delamination, usually appearing between eight and eighteen months after decoration. By this point it looks like a paint failure, and some contractors will suggest repainting as the solution. It is not. The solution is allowing the substrate to dry properly before applying any coat.
Once plaster is fully dry:
- Apply a mist coat — one part emulsion to four to five parts clean water — and allow to dry fully (typically 24 hours)
- Apply two finish coats of the specified emulsion, allowing the stated drying time between coats
The mist coat is not decorative. It penetrates the porous plaster surface and provides a stable, even base for the finish coats. Without it, full-strength emulsion sits on the surface with insufficient key and is susceptible to mechanical disruption.
Plasterboard: Different From Plaster, Different Preparation
New plasterboard (unskim) on internal walls or ceiling is common in extensions — particularly on stud walls, in the roof, or where direct boarding rather than plastering has been chosen. Plasterboard behaves differently from plastered surfaces: the board itself is non-absorbent, but the paper facing is, and the joints and screw heads need specific attention.
Tape and jointing. All joints between boards and all screw heads should be taped and jointed with a setting-type jointing compound (British Gypsum Gyproc Easi-Fill or equivalent), then sanded flush before any decoration. Joints that are only filled once will crack within the first year as the building moves; a tape-and-fill system resists this movement.
Primer coat. New plasterboard should receive a specific plasterboard primer (British Gypsum Gyproc Drywall Sealer) before emulsion is applied. Standard emulsion applied directly to new plasterboard absorbs unevenly — the paper facing and the joint compound absorb at different rates — and the finish is patchy.
MDF Joinery: The Edge Problem
New fitted furniture, built-in wardrobes, and bespoke kitchen or utility joinery in extensions frequently use MDF as the substrate material. MDF has two distinct painting challenges:
Edge absorption. The cut edges of MDF are far more absorbent than the flat faces, because the machining process opens up the fibre structure. Without edge sealing, topcoat applied to an MDF edge is absorbed and produces a rough, furry finish that cannot be resolved by additional coats alone. The correct approach is to seal all MDF edges with either neat PVA (diluted 1:1, applied twice and allowed to dry) or a dedicated MDF primer before any topcoat system.
Primer selection. Standard alkyd or water-based primers do not fully control MDF absorption on the faces either. Use a high-build MDF primer (Zinsser BIN or an acrylic MDF primer) for the flattest and most consistent topcoat finish. On site-painted fitted furniture, this preparation step separates professional results from amateur ones.
Matching Existing Rooms: The Colour and Texture Problem
Most London extension projects involve decorating a new room or open-plan space that connects directly to existing rooms. Matching the existing decoration is harder than it looks.
Colour. Even if you have the original paint tin, the old paint on the existing walls will have aged, faded slightly, and picked up a patina that fresh paint does not have. The closer the junction between old and new, the more visible this difference. The most reliable approach is to repaint the affected walls in the original colour at the same time as decorating the extension — not just the new work.
Texture. Old plastered walls and new plastered walls almost never have exactly the same surface texture, even from the same plasterer. If the junction between old and new plaster is on a visible wall rather than hidden in a corner, consider lining the old wall surface with a fine-grade lining paper (Erfurt Mav plain lining, 1200-grade) to normalise the texture before painting.
External Decoration on New Extensions
New external brickwork needs no decoration, but new render or insulated external wall systems (EWI/ETICS) do. The rule for new render is the same as for new plaster: allow it to cure fully before painting. For sand-and-cement render, this is a minimum of four weeks. For a proprietary EWI render, follow the manufacturer's specific instructions, which typically specify a waiting period and a primer system compatible with the render type.
Never apply a standard masonry paint directly to a proprietary EWI top coat without confirming compatibility. The consequences — delamination and potentially voiding the system warranty — are expensive to resolve.
Getting the Sequence Right
For a typical rear extension with a new kitchen, the correct decorating sequence is:
- Ensure all plaster is fully cured before booking the decorator
- Decorator surveys the substrate and identifies any areas of concern before quoting
- Mist coat on all new plaster surfaces; primer on plasterboard and MDF
- Fill and sand all surfaces
- Finish coats on walls and ceilings
- Woodwork system (primer-undercoat plus two topcoats) on all joinery
Skipping step one or step three is where the problems begin.
Contact us to discuss your extension project or request a free quote for properly specified extension decoration.