Future Homes Standard 2026: What It Means for Extensions and Renovations

The Future Homes Standard (FHS) is a set of changes to the Building Regulations designed to ensure that new homes produce 75–80% fewer carbon emissions than homes built under the pre-2022 regulations. Announced in 2019, the FHS is now in effect for new-build dwellings in England through updated Parts L (energy efficiency) and F (ventilation).

The standard covers insulation, heating systems, ventilation, and airtightness. For new-build homes, it means no fossil fuel boilers as the primary heating system — replaced by heat pumps or other low-carbon alternatives. For extensions and renovations on existing homes, the changes focus primarily on insulation standards, glazing, and ventilation rather than mandating a heating system change.

The full Future Homes Standard applies to new-build dwellings. However, the Part L uplift that took effect in June 2022 already applies to all building work — including extensions, loft conversions, and renovations. This means your extension must meet stricter energy efficiency standards than it would have a few years ago.

What has already changed (Part L 2021, in force since June 2022)

Higher insulation standards: The U-values (a measure of heat loss — lower is better) required for new building elements in extensions are:

In practice, this means thicker insulation in walls and floors, and higher-specification windows and doors. For wall construction, you may need 100–120mm of rigid insulation board rather than the 50–75mm that was common previously.

Impact on design: The thicker insulation layers reduce the internal dimensions of your extension slightly. A wall with 100mm of external insulation and 100mm of internal insulation is 200mm thicker than one with 50mm — which means roughly 200mm less room width and depth internally. Your architect or designer should account for this from the start.

Better windows and doors: The 1.4 W/m²K requirement means standard double glazing may no longer comply in all configurations. Many homeowners are moving to triple glazing, which comfortably meets the requirement but costs 10–20% more than standard double glazing.

Improved airtightness: New building work must be more airtight than before. Junctions between the extension and the existing house, and around windows and doors, must be carefully sealed. This has implications for ventilation — see below.

Ventilation changes (Part F)

Stricter airtightness standards mean better ventilation is needed to prevent condensation and maintain air quality. The updated Part F requires:

• Trickle vents in all new windows (larger capacity than before)
• Mechanical extract ventilation in kitchens and bathrooms (required flow rates have increased)
• Background ventilation provisions for all habitable rooms
• In very airtight extensions, mechanical ventilation with heat recovery (MVHR) may be recommended

The full Future Homes Standard is now in effect for new-build homes. Key points for 2026:

• New-build homes: Must meet the FHS energy target — effectively 75–80% lower carbon than pre-2022. Gas boilers cannot be the primary heating system in new builds. Heat pumps are the dominant solution.
• Extensions on existing homes: The Part L 2021 standards (in force since June 2022) still apply. There is no requirement to upgrade the existing boiler when building an extension — if the extension uses the existing heating system, no heating change is mandated.
• Solar PV: Required on new-build homes under the FHS. There is no current requirement to add solar panels when building an extension to an existing home.
• Future further uplift: The government may introduce stricter requirements for work on existing homes in future. Monitor Part L and the Heat and Buildings Strategy for updates.

The stricter Part L requirements do increase costs compared to a few years ago:

Insulation: Additional insulation material costs approximately £5–£15 per m² more than the previous standard. For a 20m² extension, that is roughly £200–£600 extra for walls and floors combined.

Windows and doors: Upgrading from standard double glazing to high-performance double glazing or triple glazing adds approximately 10–20% to window and door costs. For a typical extension with 3–5 windows and bi-fold doors, this could be £500–£2,000 extra.

Ventilation: Upgraded trickle vents and higher-capacity extract fans are marginally more expensive — £200–£500 extra for a typical extension.

Overall impact: For a typical single-storey rear extension, the Part L uplift adds approximately £1,000–£3,000 to the total project cost. This is a small proportion of the overall budget and is offset by lower energy bills over the life of the extension.

• Do not try to avoid the new standards. They are mandatory for all building regulations applications submitted since June 2022. Your building control officer will check compliance.
• Choose your windows early. The U-value requirement of 1.4 W/m²K means you need to check the specifications of your chosen windows before ordering. Not all standard double-glazed units meet the requirement — check the manufacturer's certificate.
• Allow for thicker walls in your design. Discuss insulation build-ups with your architect from the start. The internal dimensions of your extension will be slightly smaller than the external dimensions suggest because of the insulation layers.
• Consider the long-term benefits. A better-insulated extension is cheaper to heat, more comfortable in summer, and adds value to your property. The extra cost is typically recovered through energy savings within 5–10 years.