Energy-Efficient Extensions in Wimborne: Part L Guide

If you're planning an extension in Wimborne this year, the energy rules have moved on, and they'll move again. The new edition of Approved Document L was published in March 2026 and comes into force in March 2027. Most of it is aimed at new-build houses, but the direction is obvious: better fabric, lower-temperature heating, and a lot more paperwork if you get the glazing wrong. We build extensions across Wimborne, Colehill, Merley and Corfe Mullen, and this is what we tell clients who ask how to do it properly rather than just scrape a pass.

What Part L actually asks of a Wimborne extension

An extension isn't judged like a new house. Building Control (Dorset Council, or an approved inspector) checks the new fabric against fixed limits rather than running your whole home through a SAP calculation. The current limits for a domestic extension are:

• New external walls: 0.18 W/m²K

• Floors: 0.18 W/m²K

• Flat or pitched roofs: 0.15 W/m²K

• Windows, doors and rooflights: 1.4 W/m²K (a decent double-glazed unit gets there)

The trap is glazing area. If your windows, doors and rooflights add up to more than 25% of the extension's floor area, U-values alone won't get you through. You'll need a SAP comparison showing the proposed room performs no worse than a notional one, and very often an air test. A pair of big bifolds on a 20 m² kitchen extension tips over that line easily. We flag it at design stage, because fixing it after the frames are ordered gets expensive.

Fabric first: where the money is well spent

Insulation is cheap compared with everything else on the job. Getting 100 mm PIR board into the floor and the wall build-up of a new extension typically costs £30 to £60 per m² installed, and it's the one decision that fixes your heating bill for the next forty years. We'd rather over-insulate and use a slightly smaller rooflight than the other way round. Nobody has ever rung us to complain their extension was too warm in January.

Triple glazing is the other question we get asked. On a south-facing rear extension in Merley, good double glazing at around 1.2 W/m²K is usually fine. North-facing, or with a big glazed gable, triple starts to earn its keep. Budget £180 to £360 extra per window over a comparable double-glazed unit. It's also noticeably quieter, which matters if you back onto Leigh Road or the A31.

Heating an efficient extension

Once the fabric is right, the heat demand is low, and that changes what you should install. Wet underfloor heating running at 35 to 45°C flow temperature is the natural fit: it's comfortable, it frees up wall space for the kitchen, and it's heat-pump ready if you swap the boiler later. In a new extension with a fresh screed we install it for £120 to £170 per m², and it goes in during the build rather than as an afterthought.

One caveat. An extension zone on underfloor heating needs its own manifold and controls so it doesn't fight the radiators in the rest of the house. We've fixed a couple in Colehill where the previous installer tied it into the existing circuit and the room never got warm.

What a well-insulated extension costs in Wimborne

Based on current south-coast rates, a single-storey extension built to the standard above runs from about £2,400 per m² to around £3,100 per m², with the top end covering higher-spec glazing, underfloor heating and a full kitchen fit-out. On top of that, allow for:

• Air tightness test, where the glazing ratio triggers one: £240 to £420

• SAP comparison for over-glazed designs: a few hundred pounds from an assessor, which we arrange

• Triple glazing uplift: £180 to £360 per window

Older properties near the Minster and in the conservation area cost more, and we'd rather say so up front. Cob and solid brick walls need careful junction detailing to avoid cold bridges, and the planners will care about window profiles. We always survey before quoting, and our extension quotes list the insulation spec and target U-values line by line so you can see exactly what you're paying for.

What changes in 2027, and why it matters now

The 2026 edition of Approved Document L takes effect on 24 March 2027. The headline changes target new dwellings, with heat pumps and solar becoming the default, and the fabric limits for extensions aren't expected to shift dramatically. But building to today's minimum means owning the least efficient room in the house when you come to sell. EPC ratings carry more weight every year, and an extension detailed to the tighter new-build target values now costs very little more than one that just clears the bar.

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Planning an extension in Wimborne?

We handle design input, Building Control liaison, structure, insulation, underfloor heating and finishing under one contract across Wimborne, Colehill, Merley, Corfe Mullen and the surrounding villages. Book a free site survey through completebuilds.com and we'll walk the plot with you, talk through glazing and heating options, and give you a fixed, itemised quote.