House Extension London: A Plastering and Finishing Guide for Homeowners

Most London homeowners spend months planning their house extension — the architect drawings, the structural engineer, the builder — and then, once the roof is on and the walls are up, they suddenly realise they haven't thought about what comes next. The plastering and finishing phase is where a raw shell of blockwork, timber frame and plasterboard becomes a room you can actually live in.

If you're mid-way through a house extension in London, or just starting to plan one, this guide explains exactly what plastering work is involved, what it costs, how long it takes, and what to look out for. London Plastering Service is a specialist plastering contractor — we don't build extensions, but we finish them. Plasterboarding, dry lining, ceiling plastering, skimming, rendering: that's our part of the job, and it's the part that determines how good the finished room actually looks.

Rough cost for extension plastering in London: £700–£2,500+ depending on the size of the extension, ceiling complexity, and whether external rendering is required. We cover costs in detail further down.

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What Plastering Does a House Extension Need?

A standard house extension in London — whether it's a rear extension, side return, or kitchen extension — typically requires several distinct plastering and finishing tasks:

• Plasterboarding all internal walls and ceilings (unless blockwork is to be left and rendered)
• Dry lining external walls, often incorporating insulated board to meet Building Regulations thermal requirements
• Ceiling plastering including any vaulted sections, Velux rooflight surrounds, or lantern roof frames
• Skimming all plasterboarded surfaces to a smooth, paint-ready finish
• External rendering if the extension has a rendered exterior finish
• Patching and blending where the new extension meets the existing house

Not every extension needs all of these — a brick-faced extension with a flat ceiling is simpler than a vaulted kitchen with a glazed lantern — but most London extension projects involve at least plasterboarding, skimming, and some ceiling work.

For a full overview of what we offer, visit our extension plastering services in London.

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The Plastering Stages of a House Extension

1. Plasterboarding

Once the structural work is complete and the extension is watertight, the first plastering task is boarding. Plasterboard is fixed to the internal faces of walls and ceilings, forming the surface that will later be skimmed. In extensions, this often means working around awkward structural elements — steel beams, roof purlins, trimmer joists around rooflights — and cutting boards to fit precisely.

For extensions with exposed blockwork or brick internal walls, dot-and-dab plasterboard adhesive is commonly used. Timber-framed extensions typically use screwed plasterboard onto the studwork.

2. Dry Lining

External walls in a new extension are almost always dry lined. This involves fixing insulated plasterboard (PIR-backed board) directly to the masonry, creating both the thermal insulation layer required by Building Regulations and the surface for skimming. Getting this right matters — poorly installed dry lining can create cold bridging or uneven wall finishes.

3. Ceiling Plastering

This is often the most technically demanding part of an extension plaster job. Flat ceilings in extensions are straightforward enough, but many London kitchen extensions now feature vaulted ceilings, Velux rooflights, or glazed lantern roofs — all of which add complexity to the boarding and skimming process.

4. Skimming

Once all boards are fixed, the entire surface — walls and ceilings — is finish skimmed with a thin coat of finishing plaster. This is the layer that produces the smooth, flat finish you see when the room is painted. A good skim on an extension should be completely flat, with sharp, clean angles where walls meet ceilings, and crisp reveals around window and door openings.

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Real Project: Kitchen Extension Ceiling Replacement, Brent NW10

Patricia O. contacted us after her kitchen extension in Brent NW10 was structurally complete. The extension featured a glazed lantern roof, and the ceiling boarding needed to run flush around the entire frame of the lantern — a job that requires careful templating of each board section before cutting.

"The team boarded out the entire ceiling around the lantern roof and skimmed the walls without touching any of my kitchen units. I was worried about the angles around the glass frame but they templated every section before cutting and everything sat flush." — Patricia O., ★★★★★

The project took three days and cost in the region of £900–£1,400. See our kitchen extension ceiling case study in Brent for the full details.

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Real Project: Kitchen Extension Ceiling Plasterboard & Skim, Lambeth SW8

Helen B. in Lambeth SW8 had a kitchen extension with a vaulted ceiling incorporating two Velux windows. The vaulted geometry means every board has to follow the pitch of the roof, and the reveals around the Velux frames need to be perfectly square to look right once painted.

"Really pleased with how the ceiling came out. The vaulted section with the two Velux windows was what I was most worried about — it's not a straightforward job — but the finish across the whole ceiling is flat and the angles around the rooflights are crisp." — Helen B., ★★★★★

The project was completed in two days at a cost of £700–£1,100. See our extension ceiling plastering case study in Lambeth.

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Extension Ceiling Plastering: Vaulted Ceilings, Lantern Roofs and Velux

The ceiling is almost always the most challenging part of an extension plaster job, and it's the one that makes the biggest visual impact. Get it wrong and every painted surface will show the imperfections; get it right and the room looks finished and considered.

London kitchen extensions built over the last ten years overwhelmingly favour one of three ceiling types:

Flat ceilings with downlighters — the most straightforward to board and skim, though the plasterer still needs to work around any steel beams that might drop below the ceiling line.

Vaulted or pitched ceilings — these follow the line of the roof, often incorporating Velux rooflights. Boarding a vaulted ceiling means cutting every sheet to fit the pitch, and skimming it means working overhead on a slope — technically more demanding and typically priced accordingly.

Lantern roofs — a glazed structural frame sits within the ceiling plane. Boarding runs up to the frame on all four sides, and every angle where the plasterboard meets the frame needs to be precise. As Patricia O.'s job in Brent illustrates, this is done by templating each section before cutting, rather than cutting and hoping.

When quoting for extension ceiling plastering in London, we always assess which ceiling type is involved before giving a price. Vaulted and lantern ceilings take longer and require more material wastage, both of which are reflected in the cost.

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Extension Wall Plastering and Dry Lining

Walls in a new extension are typically either blockwork (solid masonry) or timber studwork, and the plastering approach differs slightly between the two.

Blockwork walls are either dot-and-dabbed with plasterboard or, on external walls, fitted with insulated dry lining board. The insulated board serves two purposes: it brings the wall up to the thermal performance required under Part L of the Building Regulations, and it provides a flat, stable surface for skimming.

Timber stud walls — common in lighter timber-frame extensions or as partition walls within an extension — are boarded directly with plasterboard screwed to the studs. The key here is ensuring the studwork is truly plumb and flat before boarding begins; any irregularity in the frame will telegraph through the finished plaster.

Blending new and existing walls is a task that catches many homeowners off guard. Where the extension opens into the existing house, there's almost always a junction between new plasterboard and old plaster. A good plasterer will feather the skim into the existing surface so that the join is invisible once painted.

For more information on wall finishing options and pricing, see our guide to plastering prices in London.

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External Rendering for Extensions

Not every London house extension is rendered externally — many use facing brick or timber cladding — but rendered extensions are common, particularly where the extension matches a rendered existing house, or where a contemporary white render finish is desired.

External render on a new extension is a separate scope of work from the internal plastering. It involves applying a base coat render to the external blockwork, allowing it to cure, and then applying a finish coat — either a through-colour silicone render or a monocouche product that is coloured to match the client's specification.

If your extension requires external rendering, this is typically quoted separately from the internal plastering work. The condition of the blockwork, the chosen finish system, and the number of external faces all affect the price.

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How Much Does Extension Plastering Cost in London?

Extension plastering costs in London vary depending on the size of the space, the complexity of the ceiling, and what specific tasks are required. Based on our recent extension projects, here are typical ranges:

| Scope | Typical Cost Range | |---|---| | Small extension ceiling skim (flat, under 10m²) | £400–£700 | | Medium kitchen extension — ceiling and walls | £700–£1,400 | | Vaulted or lantern ceiling — boarding and skim | £900–£1,800 | | Full extension plasterboard and skim | £1,200–£2,500+ | | External rendering (per extension face) | £500–£1,500+ |

These are indicative figures based on London pricing in 2024–2025. The actual cost depends on site access, ceiling height, the number of internal partitions, the finish specification, and whether the extension is still under construction (easier access for boarding) or already fitted out (as in Patricia O.'s case, where kitchen units were already in place).

For a detailed look at how plastering is priced in London, visit our plastering prices in London page.

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How Long Does Extension Plastering Take?

Most extension plastering projects in London take between two and five working days, depending on size and complexity. The Lambeth SW8 vaulted ceiling project took two days; the Brent NW10 lantern ceiling with wall skimming took three days.

As a rough guide:

• Small extension, flat ceiling, walls only: 1–2 days
• Medium extension, vaulted ceiling or lantern roof: 2–3 days
• Larger extension with multiple rooms or external rendering: 4–7 days

Plastering also has a drying time to factor in. Freshly skimmed surfaces need 24–48 hours minimum before painting, and in a newly built extension with a lot of moisture in the structure, proper drying time is important to avoid paint adhesion problems later.

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Common Extension Plastering Problems

Extension plastering done well is invisible — it's the foundation everything else sits on. When it goes wrong, however, the problems tend to show up after decoration, which makes them frustrating and expensive to fix. Here are the most common issues we see:

Cracking at the junction between new and old — thermal movement between a new extension and the existing house can cause cracks to appear at the join. Using appropriate stop beads and allowing the structure to settle before skimming reduces this risk.

Hollow or blown plasterboard on dot-and-dab — if the adhesive dabs dry too quickly or the board is applied to dusty masonry, sections of board can lose adhesion and sound hollow when tapped. Good surface preparation before boarding prevents this.

Uneven ceiling finish on vaulted sections — vaulted ceilings are harder to skim to a consistently flat surface. This is why experience with this specific ceiling type matters.

Mould appearing under newly plastered surfaces — new extensions can be damp during construction, and sealing this moisture in behind plaster can cause mould. Ensuring the structure is adequately dry before boarding, and using moisture-resistant board where appropriate, addresses this.

Poor reveals around rooflights — Velux and lantern roof frames need square, sharp plaster reveals. Any sloppiness here is immediately visible once the room is lit from above.

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Rear Extension vs Side Return: Plastering Considerations

The two most common house extension types in London — rear extensions and side return extensions — have slightly different plastering implications.

Rear extensions are typically the larger of the two, often spanning the full width of the house and adding significant floor area. They tend to have more wall surface to line and skim, and they're where vaulted ceilings and lantern roofs most commonly appear.

Side return extensions are narrower — they fill in the alleyway to the side of a terraced or semi-detached house — and often feature a glazed or open roof structure to borrow light. The plastering scope is usually smaller in total area, but the geometry can be more complex, particularly where the side return joins the rear of the house and creates awkward internal angles.

In both cases, the plastering and finishing phase follows the same basic sequence: boarding, dry lining external walls, ceiling work, skimming, and any blending with the existing house interior. The differences are in scale and in the specific ceiling and junction details.

If you'd like advice on the finishing scope for your specific extension type, our professional plasterer in London team is happy to discuss your project before you commit to anything.

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Frequently Asked Questions

Do I need a specialist plasterer for an extension, or can any plasterer do it? Any competent plasterer can skim a flat wall. But extension plastering — particularly vaulted ceilings, lantern roofs, and Velux surrounds — requires experience with complex geometries and the patience to template and cut accurately. We'd always recommend finding a plasterer who has done similar extension work, and asking to see examples.

When in the build process does the plastering happen? Plastering takes place after the extension is watertight (roof on, windows and doors fitted) and after first-fix trades — electrics and plumbing — have been completed. Second-fix trades (sockets, radiators, kitchen installation) typically follow after the plaster has dried.

Can you plaster around my kitchen units if they're already installed? Yes — we did exactly this for Patricia O. in Brent NW10. It requires more care and time, but it's entirely achievable. We always protect units, worktops, and appliances before starting work.

Should extension walls be wet plastered or dry lined? Most modern London extensions are dry lined (plasterboard fixed to the wall, then skimmed) rather than traditionally wet plastered. Dry lining is faster, offers better thermal performance on external walls when insulated board is used, and produces an equally good finish.

How do I get a quote for extension plastering? Get in touch with us via the contact form or phone. We'll arrange a site visit to assess the extension, take measurements, and provide a detailed written quote covering all the plastering and finishing work required. There's no obligation and no hard sell.

Will the plaster crack after the extension settles? Some minor hairline cracking at junctions between new and old is normal as a new extension settles through its first winter. Structural cracking is different and a builder's concern, not a plastering one. Using appropriate movement beads at the new-to-old junction minimises settlement cracking.

Can you match the existing plaster finish in our house? In most cases, yes. We assess the existing plaster finish before starting and aim to skim the new extension surfaces to a consistent standard. Where the existing walls are textured or have an unusual finish, we'll discuss the best approach with you before starting.

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Ready to Finish Your London Extension?

If your house extension in London is approaching the plastering stage — or if you're still in the planning phase and want to understand the finishing costs — London Plastering Service is here to help. We're a specialist plastering contractor working across London, with direct experience of extension ceiling plastering, dry lining, and skimming on rear extensions, side returns, and kitchen extensions of all shapes and sizes.

Take a look at our extension plastering services in London, browse our plastering prices in London, or get in touch to arrange a free site visit and quote. We're based in London and work throughout the city — from Lambeth to Brent and everywhere in between.