Extensions
Rendering a New Extension and the Old House Together
An extension is often the moment people decide to re-render the whole house, so the new rooms and the old walls end up under one colour. Behind that single finish sit two very different backgrounds: fresh blockwork that has never been rendered, and old masonry that may carry layers of render, paint and repairs. This guide explains what happens to each, and to the joint between them.
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New blockwork: a clean start with its own rules
Extension walls are usually concrete blockwork built to be rendered, the kind of background through-coloured systems are designed around. In effect this is new-build rendering attached to an old house. K Rend lists its K Mono monocouche for moderate to low strength blockwork, applied in one pass to about 17-18 mm and scraped back once it has set, which its guidance puts at anywhere between 3 and 24 hours depending on the background and drying conditions. Reveals and quoins can be finished smooth for contrast.
New masonry needs time before render goes on. Lime Green's guidance for its lime renders asks for new masonry to have cured for 28 days, and cement-based systems give their own figures in the product sheets. The blockwork should be clean of mortar droppings, and the base bead should sit above the extension's own damp-proof course, which may not line up with the course in the old house.
Because the extension is new, its frames, sills and roof edges can be set out with the render in mind: sills that project far enough to throw water clear of the finished face, fascias and verges that cover the top edge of the render, and downpipe positions agreed before the finish goes on.
The old house: strip, repair and build back to the same face
The original walls are a different job. The old render is sounded and stripped where it is hollow, cracked or poorly bonded; K Rend's guidance for its products is that an existing layer comes back to a stable substrate before re-rendering. The masonry underneath may be brick, stone or both, with old repairs and filled openings.
Before any finish goes on, the old walls have to be brought out to a true face. That usually means dubbing out hollows with a base coat (K Rend's guide describes its UF Base as ideal for dubbing out), a primer where the brick is very absorbent or offers little key (the guide lists R7 and R10 primers for those conditions), and alkali-resistant mesh at reveals and other stress points. Only then can one finish run over both parts and read as the same wall.
If the original house is solid stone or brick that was built for lime, the old walls may need a lime system for the reasons SPAB gives about cement trapping moisture. The extension can still be finished to look close to it, but the two parts will carry different materials behind a similar colour, and that difference belongs in the handover notes.
The joint where new meets old
The junction between the extension and the original house is usually where trouble shows first. The two structures are built at different times on different foundations, and they settle and move differently. A single sheet of render run straight across that line is liable to crack along it.
The usual answers are a movement bead or a deliberate stop at the junction, mesh lapped well past the change of background, or a visual break such as an internal corner or a slight step that makes the line read as intended. Which suits depends on the system and the structural detail, so it belongs on the drawings rather than being settled on the scaffold.
Rainwater fittings tend to gather here too: a new downpipe from the extension roof, a valley gutter, a soil pipe moved for a new bathroom. Fix their positions before rendering, so brackets go into the finished wall once. K Rend's maintenance advice notes that rusting brackets stain render, so stainless fixings are worth specifying.
A flat-roofed extension adds one more edge. Where the render meets a parapet or a roof trim, the top of the render needs a coping or trim that projects with a drip, so rain running off the roof is thrown clear instead of soaking into the top few centimetres of the new coat. The same applies to any new window that sits in the junction wall: its sill should project past the finished face of whichever system is used on that side.
Planning, Part L and the order of work
An extension has its own planning position, and re-rendering the original house is a separate change. In conservation areas and the National Park, the permitted development rules exclude cladding any part of a house with render, so a whole-house change of finish there needs checking with the planning authority alongside the extension drawings.
Approved Document L treats the two parts differently as well. The extension is new work with its own energy standards, while re-rendering the existing walls counts as renovation: if more than half of an existing wall element is renovated, the guidance expects it to be upgraded where that is feasible. On a traditional stone house, paragraph 0.10 says improvement should only go ahead if it will not cause long-term deterioration of the fabric.
A sensible order is to build the extension shell, let its blockwork cure, strip and repair the old walls during that wait, then base-coat both and finish both in one programme so the colour goes on in the same season. As a hypothetical example, on a 1930s semi gaining a rear kitchen, the old pebbledash could come off while the kitchen roof is going on, leaving both ready for base coat together. If the wider scope is still taking shape, start with the whole-house overview on the homepage.
Common questions
Should the extension use the same render system as the house?
Not necessarily. The finish coat, colour and texture can be the same product while the preparation and base coats differ, because new blockwork and old masonry need different treatment. What matters is that the visible finish is applied as one programme.
Will a monocouche on the extension match a silicone finish on the house?
Not reliably, even in similarly named colours, because they are different products with different textures. If the two parts must read as one, use one finish product across both, or make test panels of each side by side; K Rend's Silicone TC sheet recommends a test panel before the main work.
When can new blockwork be rendered?
When it has cured for the period the chosen product requires. Lime Green asks for 28 days on new masonry for its lime renders; cement-based systems state their own figures. Check the data sheet for the system actually specified.
See housing and planning information for Sheffield and nearby places, including the source and geography behind each figure.
