Net zero rules will make home extensions smaller and more expensive

You might have noticed things getting both smaller and more expensive - and now this is set to happen to extensions too.

Net zero rules mean floor space is set to give way to thicker insulation as new building standards set tighter restrictions on energy efficiency.

Estimates for The Telegraph suggest this could reduce extension sizes by several feet, as space-poor homeowners have limited room to extend.

Britain's homes tend to be poorly insulated, with less than half having cavity or solid-wall insulation, and the new regulations are part of a push to cut energy use.

Rules governing the thermal efficiency of walls have been tightened, meaning homeowners must either add thicker insulation or splash out on cutting-edge alternatives that insulate walls without adding too much thickness.

The Government estimates that the change will increase wall thicknesses by around 1.5 inches, while experts contacted by The Telegraph said an extra two inches was likely.

Gareth Belsham, director at consultancy and surveyor Naismiths, said: “Extensions will have smaller or less window space than before, and those windows will be very, very highly insulated, as will the walls.

“So in actual fact it’s equivalent to losing a bit of floor space. In order to get the same floor space, ironically you’ve got to build more."

The new rules are part of an overhaul of building standards to make British property more energy-efficient, easier to heat in the winter and less likely to overheat in the summer.

Jonathan Rolande, founder of the National Association of Property Buyers, said homeowners building on a restricted patch of land in urban areas were most likely to be affected.

"The square footage remains the same and if the walls are thicker it could potentially impact on the room size," he said.

"That’s going to be an issue particularly in cities and London particularly, where people are really squeezing as much as they can onto the smallest space, because in cities there are more issues with boundaries and the value of space itself.”

Estimates suggest a typical extension was also likely to become around £3,000 more expensive, based on the increased cost of triple-glazing and insulation, though homeowners would recoup their money more quickly through lower energy bills, he added.

Mr Belsham said one inch extra of internal insulation would reduce a typical 13 by 13 foot-room by over four square feet.

Separate estimates by architect Andrew Mellor, partner at consultancy PRP, suggested a reduction of around six square feet of floor space on an extension measuring 3 metres by 4 metres (9.8 feet by 13.1 feet), a standard size for a semi-detached house.

Rico Wojtulewicz, head of housing and planning policy at the National Federation of Builders, said: “Many people are buying later in life, and they're going to be the most impacted by any additional cost, because they've probably stretched their budget to afford the house in the first place.

"The cost to give up your mortgage, if you're on a fixed rate, moving out, is a disincentive. So they'll be looking to have an extension instead.

"They'll be looking for the absolute maximum they can for their money, so they're going to be looking at cost-cutting measures, and they're going to be the ones hit hardest. That's the real concern.

"It's not a bad thing, because you need more energy-efficient homes, but it's clearly something that will impact people."

A provision in the regulations also allows for a lower standard of insulation if more than five per cent of the size of a room is likely to be lost.

A spokesman for the Department for Levelling Up, Housing and Communities said: “We do not recognise these calculations – the impact on room size will be minimal and these changes will save households money on their fuel bills.”