Future Homes Standard: What It Means for Extensions in 2026
The Future Homes Standard (FHS) became the default English building-regulations baseline on 15 June 2025, with full enforcement in force across 2026. While headlines focus on new builds, the rules also reshape how extensions are designed, heated and certified. This guide covers exactly what changes for a 2026 extension project — and what it adds to your build cost.
What Changes for Your Extension Under FHS
FHS extends Part L 2025 fabric and services standards to most new heated work — including extensions over 10 m². The five biggest practical impacts are below.
New radiators in extensions cannot extend a gas system unless the host dwelling already meets fabric-first criteria. Most projects now spec heat-pump-ready pipework (28mm flow/return) and low-temperature emitters.
Aggregate U-values for new extension elements drop noticeably vs Part L 2021. Practical impact: 150 mm PIR floor, 140 mm PIR wall, 250 mm loft / 170 mm warm-roof PIR.
Double-glazed argon units rarely meet 1.2 W/m²K once frames are factored in — most 2026 specifications now default to triple-glazed aluminium-clad timber or quality uPVC.
Pre-2025 architects and builders are still catching up. We match you with vetted FHS-trained designers and contractors who price compliance correctly first time.
Get matched →FHS Extension Compliance vs Part L 2021
| Element | Part L 2021 | FHS 2026 | Cost Uplift |
|---|---|---|---|
| External wall U-value | 0.26 W/m²K | 0.18 W/m²K | +£35 / m² |
| Roof / loft U-value | 0.16 W/m²K | 0.15 W/m²K | +£8 / m² |
| Window U-value | 1.4 W/m²K | 1.2 W/m²K | +£90 / m² |
| Heating circuit | Extend gas OK | Heat-pump-ready | +£1,800 lump |
| Overheating (Part O) | Limited | Mandatory check | +£350 fee |
The Five Things That Actually Change On Site
1. No new fossil-fuel heating circuits
You can keep your existing gas boiler, but you cannot extend the gas main into a new extension unless the project is part of a like-for-like replacement. Practically this means designers either: (a) extend the existing wet system but spec low-temperature emitters and 28mm pipework so the dwelling can switch to a heat pump later, or (b) install electric infrared / underfloor electric for the extension only.
2. Fabric-first U-values across all new elements
The numbers in the table above are not negotiable for new construction. Existing party walls being upgraded as part of the extension only need to meet the lower 0.55 W/m²K refurbishment standard, but anything genuinely new (a rear wall, a new pitched roof, a new ground-bearing slab) is held to FHS levels.
3. Mandatory overheating analysis (Approved Document O)
Any extension with significant south- or west-facing glazing must run a Part O assessment — usually a TM59 or simplified-method calculation. Result: solar control glass, openable rooflights with restrictors, or external shading is now common-spec on rear extensions with bi-folds.
4. PV-ready roofs where viable
FHS does not mandate solar PV on extensions, but it does require the design to demonstrate that PV was considered on any roof pitch above 15° that receives reasonable solar gain. Most building-control bodies expect either an installed array or a structurally-reinforced future-PV zone.
5. SAP-aggregate calculation for the whole dwelling
Your extension's energy balance is calculated in aggregate with the host home. If the host has an EPC F or G, a high-spec extension can be allowed; if the host is already efficient, your extension must not drag the SAP down — a simpler outcome for most projects.
What FHS Adds to a Typical 30 m² Extension
For a 30 m² single-storey rear extension, FHS-compliance against the previous Part L 2021 baseline typically adds £4,200 to £9,800 depending on glazing area, heating choice and existing heating system. The lower end assumes a small extension with limited glazing and a heat-pump-ready electric loop; the upper end assumes a fully glazed garden room with triple glazing, MVHR and PV.
Worked example: 30 m² rear extension, Manchester
- Wall area 24 m² × £35 = £840
- Roof area 30 m² × £8 = £240
- Glazing 12 m² × £90 = £1,080
- Heat-pump-ready pipework / emitters = £1,800
- Part O analysis = £350
- PV-ready provision (cabling + reinforced zone) = £600
- FHS uplift total: £4,910
This sits inside the typical £55,000 to £75,000 build budget for a 30 m² Manchester extension.
Future Homes Standard for Extensions — FAQ
Does FHS apply to my extension if planning was approved before 15 June 2025?
If you submitted the building-regulations application before 15 June 2025 and start on site within 12 months of approval, you can build under Part L 2021. Any application submitted on or after that date is FHS.
Can I still install a new gas combi boiler in 2026?
Yes — like-for-like boiler replacement remains permitted. What changes is that you cannot install a brand-new gas circuit into a new heated extension under FHS without the wider system being heat-pump-ready.
Do I need MVHR in a new extension?
Not mandatory, but FHS air-tightness expectations (5 m³/h·m² vs 8 previously) make natural-ventilation strategies harder. About 60% of 2026 extension designs now spec dMEV or MVHR.
Will FHS make my extension more expensive overall?
Yes — typically 6 to 12% on a £55k–£75k 30 m² project — but operating-cost savings of £180 to £420 per year and a higher resale EPC offset the uplift inside 8 to 14 years.
Does FHS apply to garage conversions?
Yes for the heated and habitable element, including the floor build-up, wall insulation and any new glazing. Existing structure can stay if the U-value can be brought to refurbishment standard (0.30 wall / 0.28 roof / 0.25 floor).
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