James Whitaker
Start with permissions, not with a digger
It is tempting to book a builder the moment you have sketched your dream extension on the back of an envelope. Resist that. On a single storey rear extension there are usually three separate approvals running in parallel, and they move at very different speeds. Getting them wrong is the single most common reason a project stalls in the first fortnight.
Planning comes first. Many single storey rear extensions fall under permitted development, typically up to 3 metres beyond the rear wall of an attached house, or 4 metres for a detached one, with height limits and the neighbour consultation route applying to the larger sizes. If your house is listed, in a conservation area, or already has its permitted development rights removed, you will need a full application. Either way, get the position confirmed in writing before you commit to a design.
Then there is the Party Wall etc. Act 1996. If you are excavating within 3 metres of a neighbour's foundations and going deeper than them, or within 6 metres on a deeper excavation line, or building on or against a party wall, you must serve notice. Notice periods are typically two months for building works and one month for excavation, so this is not something to leave until the week before the digger arrives.
Finally, Building Regulations. Submit a full plans application or a building notice, and expect inspections at excavation, foundation, damp proof course, drainage, and structural opening stages. Miss an inspection and you can end up exposing work at your own cost.
Nail down the design and specification
Vague drawings produce vague quotes, and vague quotes produce budget creep. Before you go to tender, make sure you have a set of drawings that answers the awkward questions.
- Dimensions and head heights. A single storey extension with a flat roof typically gives you a ceiling height of around 2.4 to 2.5 metres internally. Check this against your proposed doors and rooflights.
- Roof type. Flat roofs with a warm deck, sedum, or a pitched roof with tiles — each has different implications for height, drainage and cost.
- Fabric specification. Wall build-up, insulation type and thickness, and U-values need to be agreed so your builder is pricing the same thing your building control body will inspect.
- Provisional sums. Groundworks, drainage and steelwork are the three areas where estimates most often drift. Ask for them to be identified separately.
A 10 to 15 per cent contingency is sensible, not pessimistic. Treat it as part of the budget rather than a sign the quote was wrong.
Foundations, drainage and everything buried
Groundworks are where the unknowns live. Foundation depth depends on soil type, the presence of trees, and how deep your existing house footings sit. Trench fill footings at around a metre are common, but clay soils with nearby trees can demand considerably deeper excavation to avoid heave. Your building control inspector will want to see the trench bottom before concrete goes in.
Drainage matters just as much. If the extension sits over or near a public sewer, you will likely need a build over agreement from your water authority, and the sewer may need to be protected or diverted. Surface water needs a plan too — a soakaway, a connection to an existing drain, or a rainwater harvesting arrangement. Confirm capacity before you dig, not after.
Superstructure and making it weathertight
Once the slab or oversite is in and the damp proof course is laid, the walls go up quickly. Cavity walls with a full-fill or partial-fill insulation build-up are the norm, and the external finish should match or deliberately contrast with the existing house. Think about the junction between new and old — a movement joint, a lead flashing, or a stepped detail will usually be needed where the new roof meets the existing wall.
Get the roof on and the shell watertight before you start cutting into the house. This is the point where a well-sequenced job protects your home from a week of rain and a very unhappy family.
Breaking through, glazing and openings
The knock-through is the moment everyone has been waiting for, and it is also the most technically demanding. Structural openings need a steel beam or lintel sized by an engineer, padstones to spread the load, and temporary propping while the wall is removed. Never let this be decided on site by eye.
Glazing lead times catch people out badly. Bi-fold and sliding doors can take six to twelve weeks, and they need to be ordered well before the opening is ready. Check the current limiting U-values with your building control body, particularly if you are planning a large amount of glazing — extensions with more than the usual proportion of glass often need compensating measures elsewhere in the fabric. Rooflights add light but also add a thermal weak point, so specify them properly.
Sequencing, programme and keeping the budget honest
A realistic programme runs roughly: permissions and party wall notices, then groundworks, foundations, drainage, slab, walls, roof, knock-through, glazing, first fix, plaster, second fix, decoration. Each stage has dependencies, and rushing one usually costs more at the next.
- Book building control inspections as soon as each stage is ready — inspectors are busy.
- Order long-lead items (glazing, steels, bespoke joinery) as early as the design allows.
- Keep a written record of variations, with costs agreed before work proceeds.
- Protect the existing house: dust screens, floor protection, and a plan for the kitchen being out of action.
Do those things and a single storey rear extension becomes a genuinely enjoyable project rather than a running argument about money.
