The number 31% appears in the new building regulations – the minimum energy efficiency improvement required for new homes from June 2023. That figure is not abstract policy. It translates into thicker insulation, triple-glazed windows, and heat pumps being mandatory in most new builds. For existing homeowners, the rules are less blunt but equally consequential.
The House of Commons Library published a detailed briefing on the changes, as reported by The House of Commons Library. The briefing covers Part L (conservation of fuel and power) and Part F (ventilation), which were overhauled in 2023. The key point for anyone replacing a boiler, fitting new windows, or adding an extension: the bar has been raised, and the cost of non-compliance is rising.
Who qualifies – and who doesn’t
The regulations apply to ‘material alterations’ – any work that changes the energy performance of a building. That includes replacing a heating system, adding or replacing insulation, and installing new glazing. If you are simply redecorating, you are fine. But if you swap out an old boiler for a heat pump, the system must meet minimum efficiency standards set out in SAP 10.2. For heat pumps, that means a seasonal coefficient of performance (SCOP) of at least 2.5 – effectively 250% efficiency. Most modern heat pumps achieve this, but the older models some installers push may not.
The catch is enforcement. Local authority building control or approved inspectors must sign off the work. If they do not, the property may fail an EPC assessment later, which can block a sale. The Energy Saving Trust estimates that around 15% of home improvements currently miss compliance on first inspection.
What it costs a typical 3-bed semi
Take a typical 1970s three-bed semi in Manchester. The owner wants to replace single-glazed windows with double glazing. Under the old rules, a U-value of 2.0 W/m²K was acceptable. The new regs demand 1.6 W/m²K – roughly a 20% improvement. That means argon-filled, low-E coated glass with warm-edge spacers. Cost: typically £4,000–£6,000 for the whole house, up from £3,000–£4,500 for basic double glazing. But the EPC gain is real: jumping from band D to C is common, which can add 2–4% to the property value according to Nationwide.
For insulation, the rules require a minimum U-value of 0.18 W/m²K for roof insulation (up from 0.20). That means 300 mm of mineral wool, not 270 mm. The extra 30 mm costs about £50 for a 50 m² loft – negligible in the scheme of a £1,500 job.
The ventilation trap
Part F now demands mechanical ventilation with heat recovery (MVHR) in any home where air permeability drops below 3 m³/h/m² at 50 Pa. That is a technical way of saying: if you insulate so well the house becomes airtight, you must install a ventilation system to stop mould and condensation. Many homeowners who add cavity wall insulation and new windows discover this only when the building inspector arrives. A whole-house MVHR system costs £2,500–£4,500 installed. Budget for it.
What to do now
Check the date of your last EPC. If it is older than 10 years, the recommendations may be based on pre-2023 standards. Ask any installer for a written statement confirming compliance with Part L 2023. For heat pumps, insist on a MCS-certified installer – they will handle the SAP calculation. The government’s Boiler Upgrade Scheme provides £7,500 off a heat pump, but only if the installation meets the new regs.
Deadline: the regulations are already in force. Any work started after 15 June 2023 must comply. If you are planning a renovation in 2025, factor in an extra £1,000–£2,000 for upgraded glazing and ventilation. The payoff is a warmer home, lower bills, and a property that sells faster.
Frequently Asked Questions
Yes. Replacing a heating system counts as a 'material alteration' under Part L. The new system must meet minimum efficiency standards – for heat pumps, a SCOP of at least 2.5; for gas boilers, at least 92% efficiency. Your installer should provide a compliance certificate.
Not automatically, but compliance typically raises the score by 5–15 points. The EPC assessment uses SAP 10.2, which aligns with the new regs. Upgrading windows from 2.0 to 1.6 U-value alone can add 2–3 points. Combining insulation, glazing, and a heat pump often lifts a D to a B.
