Future Homes Standard 2026: What It Means for London Extensions
The Future Homes Standard is coming for new builds. Extensions are not directly in scope yet, but the direction of travel is clear and the current Part L requirements already demand high-performance builds. Here is what you need to know and how to future-proof your extension.
Quick Answer
The Future Homes Standard primarily targets new-build homes, with regulations expected to come into force in late 2026. Extensions must already comply with Part L 2021, which requires walls at 0.18 W/m²K, roof at 0.15 W/m²K, and glazing at 1.4 W/m²K. Meeting current standards properly and future-proofing adds £3,000–£8,000 to a typical extension.
0.18 W/m²K
Wall U-value (current)
£3k–£8k
Extra cost to future-proof
0% until Apr 2027
VAT on energy upgrades
Check your specific property constraints
Free Property CheckWhat Is the Future Homes Standard?
The Future Homes Standard (FHS) is a major update to Part L (conservation of fuel and power) and Part F (ventilation) of the Building Regulations in England. It requires new-build homes to produce 75–80% lower carbon emissions than current standards, effectively ending gas boiler installations in new homes and mandating heat pumps, high-performance insulation, and in most cases solar PV.
The government laid the legislation before Parliament in December 2025, with regulations expected to come into force in December 2026. A 12-month transitional period will run to December 2027, after which all new homes must comply.
FHS timeline
The critical distinction: the FHS targets new dwellings. Extensions to existing homes are governed by Part L 2021 (the current standard, in force since June 2022). But the trajectory is clear. Part L requirements for extensions have tightened significantly with each update and will continue to do so.
How the Future Homes Standard Affects Extensions
The FHS does not directly apply to extensions in its initial form. Extensions must meet the existing Part L 2021 standards, which Building Control enforces on every project. However, the FHS matters for extension projects for three reasons:
1. Part L will tighten further
The FHS consultation includes proposals for work on existing buildings. When the full standard is implemented, minimum U-values for extensions are expected to drop again. Building to just the current minimum now means your extension may not meet the standards of a few years' time, which could affect resale value and EPC ratings.
2. Heating system trajectory
The FHS mandates heat pumps for new builds. While you can still extend your existing gas boiler system into a new extension, the policy direction is away from gas. Future-proofing means laying wet underfloor heating pipework in the slab (compatible with a heat pump later) even if you connect to your boiler now.
3. EPC requirements are tightening
The government intends to raise minimum EPC requirements for rental properties and eventually for sales. A well-insulated extension improves your whole-house EPC. An extension built to bare-minimum standards can lower it.
Current Insulation Standards for Extensions (2026)
Part L 2021 is the current standard enforced by Building Control for all extensions in England. These U-values (heat loss rates) are non-negotiable. Lower numbers mean better insulation.
| Element | Max U-value (W/m²K) | Typical build-up | Future-proof target |
|---|---|---|---|
| Walls | 0.18 | 100mm PIR cavity fill or 140mm timber frame | 0.12–0.15 |
| Roof | 0.15 | 150–170mm PIR above deck (warm roof) | 0.10–0.12 |
| Floor | 0.18 | 100mm PIR under screed or 150mm EPS under slab | 0.10–0.13 |
| Windows and doors | 1.4 | Double-glazed argon-filled, thermally broken frame | 0.8–1.0 |
| Rooflights | 1.7 | Double-glazed with low-e coating | 1.0–1.2 |
Common mistake: Many builders and online guides still reference the old pre-2022 wall U-value of 0.28 W/m²K. The current requirement is 0.18. If your builder's specification does not include a U-value calculation document, ask for one before work starts.
Exceeding the minimum costs relatively little during construction. The difference between 0.18 and 0.13 for walls is typically an additional 25–40mm of insulation board: £800–£2,000 for a standard extension. That small upfront cost delivers lower running costs for the life of the building.
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Heat Pumps and Extensions
The FHS mandates heat pumps for all new-build homes. For extensions, there is no current requirement to install a heat pump. Most extensions connect to the existing gas boiler system. But the extension project is the most cost-effective time to switch, for two reasons.
- Underfloor heating pipework can be laid in the new slab at minimal extra cost. Wet UFH runs at the low flow temperatures that heat pumps prefer (35–45°C vs 60–75°C for radiators). Doing this later means breaking up the finished floor: £3,000–£8,000.
- The site is already disrupted. An air source heat pump unit sits outside (typically at the side of the house). Running pipework through an open building site costs far less than routing through a finished house.
Heat pump economics for extensions in 2026
Even if you are not installing a heat pump now, lay the UFH pipework in the extension slab. It costs under £1,000 during construction and saves thousands if you switch to a heat pump in the next 5–10 years.
Not sure if a heat pump makes sense for your property? Our AI checks your address, extension plans, and grant eligibility.
Check now — it's free →Ventilation Requirements
Part F 2021 governs ventilation for extensions. Better-insulated, more airtight extensions need more considered ventilation to avoid condensation and maintain air quality. There are two main approaches.
Trickle vents (standard approach)
- No additional cost (built into window frames)
- No maintenance or running costs
- No heat recovery (warm air leaves the building)
- Not suitable for very airtight builds
Sufficient for most standard extensions
MVHR (mechanical ventilation with heat recovery)
- Recovers 73%+ of heat from exhaust air
- Filters incoming air (reduces dust and allergens)
- Install cost: £3,000–£6,000 (whole house)
- Requires duct runs (easier during a build)
Worth considering for airtight or Passivhaus-level builds
For most London extensions, trickle vents plus kitchen extract ventilation (60 l/s) meet Part F. If you are building to significantly better-than-minimum insulation levels and achieving airtightness below 5 m³/m²/h at 50 Pa, MVHR becomes the appropriate ventilation strategy and will be mandated under the FHS for new builds. Planning duct routes during the extension design stage avoids costly retrofitting.
Triple Glazing vs Double Glazing
Standard double glazing (argon-filled, low-e coating, thermally broken frame) achieves U-values of 1.2–1.4 W/m²K and meets the current Part L requirement of 1.4 W/m²K. Triple glazing achieves 0.8–1.0 W/m²K, roughly 40% better.
| Factor | Double glazing | Triple glazing |
|---|---|---|
| U-value | 1.2–1.4 W/m²K | 0.8–1.0 W/m²K |
| Cost premium | Baseline | 15–25% more |
| Weight | Standard | 30–50% heavier per panel |
| Sound insulation | Good | Noticeably better |
| Energy payback | — | 20+ years on energy savings alone |
When triple glazing is worth it
- North-facing extensions where heat loss through glass is highest and solar gain is minimal.
- Large glazed areas (3m+ bi-fold or sliding doors) where the absolute heat loss is significant.
- Noisy locations (busy roads, flight paths). The sound insulation improvement is noticeable.
- Heat pump systems. Lower heat loss means the heat pump runs more efficiently and the extension stays warmer at lower flow temperatures.
For a typical rear extension with 3m of bi-fold doors, upgrading from double to triple glazing adds £600–£1,800. The heavier panels may require upgraded hardware. The FHS is expected to require U-values of 1.2 W/m²K or lower for glazing in new builds, making high-performance double glazing the new minimum and triple glazing the logical choice for future-proofing.
Solar Panels When Extending
The FHS will require solar PV on most new-build homes, with the government specifying 40% coverage of the building's floor area where feasible. For extensions, solar PV is not required but the extension project creates an ideal window to install it.
- Permitted development covers most rooftop solar. Panels can be installed on the existing roof without planning permission (unless listed, in a conservation area, or protruding above the ridge).
- Shared scaffold saves £800–£2,000. Scaffold hire is the major cost element of a standalone solar installation. Bundling with the extension eliminates this.
- 0% VAT until April 2027. Solar PV installation on residential properties is currently zero-rated for VAT, saving 20% on the install cost.
Solar PV economics when added during extension (2026)
What This Adds to Extension Costs
Meeting current Part L 2021 standards and future-proofing your extension adds cost compared with building to the old pre-2022 standards. Here is what each element typically adds:
| Upgrade | Additional cost | Status |
|---|---|---|
| Part L 2021 compliant insulation (walls, roof, floor) | £1,500–£3,500 | Required |
| Part L compliant glazing (1.4 W/m²K) | £500–£2,000 | Required |
| Triple glazing upgrade (0.8–1.0 W/m²K) | £600–£1,800 | Optional |
| UFH pipework in slab (future heat pump ready) | £500–£900 | Recommended |
| Better-than-minimum insulation (0.12–0.14 walls) | £800–£2,000 | Recommended |
| MVHR system (whole house) | £3,000–£6,000 | Specialist only |
In total, meeting current standards adds £2,000–£5,500 to a typical extension compared with the old pre-2022 standards. Adding future-proofing (better insulation, triple glazing, UFH pipework) adds a further £1,000–£4,000, bringing the total premium to £3,000–£8,000. This is recoverable through lower energy bills and improved EPC rating at resale.
0% VAT on Energy Efficiency Improvements
Until 31 March 2027, the installation of specified energy-saving materials in residential properties is zero-rated for VAT. This saves 20% on the supply and install cost of qualifying technologies. After April 2027, the rate returns to 5%.
What qualifies for 0% VAT
The 0% rate applies to supply and installation as a single service. Materials-only purchases remain at 20% VAT. The relief applies until 31 March 2027.
Timing opportunity: If your extension project runs in 2026 and you are considering a heat pump, solar PV, or upgraded insulation, commissioning these as part of the build before April 2027 saves 20% VAT on qualifying elements. Combined with the £7,500 BUS grant for heat pumps, the effective cost of switching to a heat pump drops significantly.
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How to Future-Proof Your Extension
The cost of future-proofing during construction is a fraction of retrofitting later. Here are the measures worth taking now, ranked by cost-effectiveness.
1. Lay UFH pipework in the slab (even if connecting to gas boiler now)
Cost: £500–£900. Retrofit cost: £3,000–£8,000. This is the single most cost-effective future-proofing measure. The pipes work with your current boiler and with a future heat pump.
2. Insulate to better-than-minimum standards
Cost: £800–£2,000 above minimum. Adding 25–40mm of additional insulation board during construction is cheap. Insulation is the one thing you cannot improve without demolition once the walls and roof are finished.
3. Specify high-performance glazing
Cost: £600–£1,800 upgrade to triple. Glazing is a major source of heat loss in an extension. Triple glazing or high-performance double glazing (1.0–1.2 W/m²K) will meet future standards without replacement.
4. Install solar PV while scaffold is up
Cost: £4,500–£7,000 when bundled (saving £2,000+ vs standalone). The FHS requires solar on new homes. Getting ahead of this adds value and income from day one. 0% VAT until April 2027.
5. Consider a heat pump if your boiler is nearing end of life
Net cost after £7,500 grant: £3,000–£10,500. If your gas boiler is 10+ years old and the extension involves a new heating zone, switching during the build costs far less than a standalone conversion. 0% VAT on install until April 2027.
Frequently Asked Questions
Does the Future Homes Standard apply to extensions?
Not directly, not yet. The FHS primarily targets new-build homes. Extensions must comply with Part L 2021, which sets U-values of 0.18 W/m²K for walls, 0.15 for roofs, 0.18 for floors, and 1.4 for glazing. However, future Part L updates influenced by the FHS are expected to tighten these requirements for extensions too.
What are the current insulation standards for extensions in 2026?
Part L 2021 (in force since June 2022) requires: walls 0.18 W/m²K, flat/pitched roof 0.15 W/m²K, floor 0.18 W/m²K, windows and doors 1.4 W/m²K, rooflights 1.7 W/m²K. These are significantly tighter than the pre-2022 standards (walls were 0.28, roof was 0.18). Building Control enforces these on every extension.
Do I need a heat pump for my extension?
No. There is no current requirement to install a heat pump for an extension. Most extensions connect to the existing gas boiler system. However, laying wet underfloor heating pipework in the extension slab (£500–£900) is strongly recommended as it makes a future heat pump conversion far cheaper.
Is triple glazing worth it for an extension?
Triple glazing (0.8–1.0 W/m²K) costs 15–25% more than standard double glazing (1.2–1.4 W/m²K). It is worth it for north-facing extensions, large glazed areas, noisy locations, and if you plan to install a heat pump. The energy payback alone takes 20+ years, but the comfort, noise reduction, and future-proofing benefits make it worthwhile for many extension projects.
What is the 0% VAT on energy improvements?
Until 31 March 2027, the installation of certain energy-saving materials in residential properties is zero-rated for VAT. This covers heat pumps, solar PV, insulation, battery storage, and heating controls. After April 2027, the rate returns to 5%. This makes 2026–2027 a particularly good time to bundle these improvements with an extension project.
How much does future-proofing an extension cost?
Meeting current Part L 2021 standards adds £2,000–£5,500 to an extension compared with pre-2022 standards. Adding future-proofing measures (better insulation, triple glazing, UFH pipework) adds a further £1,000–£4,000. Total premium: £3,000–£8,000, recoverable through lower energy bills and improved EPC at resale.
Should I install solar panels during my extension?
If you have suitable south-facing roof space, yes. A 4kWp system added during the build costs £4,500–£7,000 (vs £6,500–£9,000 standalone) because scaffold is already up. Annual bill savings: £600–£900. Payback: 6–10 years. Rooftop solar is permitted development for most homes. 0% VAT applies until April 2027.
When does the Future Homes Standard come into force?
Legislation was laid before Parliament in December 2025. Regulations are expected to come into force in December 2026 with a 12-month transitional period to December 2027. After that, all new homes must meet the standard. Extensions are not in the initial scope but will likely face tighter Part L requirements in subsequent updates.
Summary
The Future Homes Standard does not yet apply directly to extensions, but the direction is clear: higher insulation, lower-carbon heating, and better ventilation. Part L 2021 already requires significantly better performance than many builders and online guides realise (walls 0.18, roof 0.15, not the old 0.28 and 0.18).
The most cost-effective future-proofing measures are those that cannot be done later without demolition: insulation beyond the minimum, UFH pipework in the slab, and high-performance glazing. These add £3,000–£8,000 to the build but reduce running costs for decades.
With 0% VAT on heat pumps, solar PV, and insulation until April 2027 and a £7,500 grant for heat pumps through the Boiler Upgrade Scheme, 2026 is an unusually good year to combine an extension with a whole-house energy upgrade.
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