Part L 2026 sets a maximum door U-value of 1.6 W/m²K for new-build and most replacement doors
If you are buying or replacing an external door in 2026, the legal limit for heat loss is set by Part L of the Building Regulations. The 2026 edition of Approved Document L specifies a limiting U-value of 1.6 W/m²K for doors that form part of the building fabric, according to the Ministry of Housing, Communities and Local Government (GOV.UK – Approved Document L, 2026 edition). This applies to external doors in new dwellings and most replacement doors in existing homes, unless the dwelling is exempt (e.g., listed buildings or small porches) (GOV.UK – Part L compliance guidance).
The U-value measures heat loss through the door in watts per square metre per degree Kelvin (W/m²K); lower is better, and 1.6 is the legal maximum for compliance. Doors with more than 50% glazing (e.g., French doors) may be treated as windows in some cases, with a separate U-value requirement of 1.8 W/m²K for the glazed area (MHCLG – Part L 2026, Section 3.12).
The actual cost of a Part L-compliant door in 2026 ranges from £500 to £2,500 installed
A basic uPVC door with a U-value of 1.4–1.6 W/m²K costs between £500 and £1,200 installed, based on industry pricing from the Glass and Glazing Federation (GGF – 2026 market price survey). A composite door (e.g., GRP or timber core) with a U-value of 1.2–1.4 W/m²K ranges from £800 to £1,800 installed (GGF – 2026 pricing data). High-performance timber doors (e.g., oak with insulated core) achieving U-values below 1.2 W/m²K cost £1,500 to £2,500 installed (Timber Trade Federation – 2026 cost estimates). Glazed door units (e.g., French or patio doors) add £300–£600 for double or triple glazing to meet the 1.8 W/m²K glazing limit (FENSA – 2026 installation cost guide).
Quick numbers U-value thresholds, costs, and savings for common door types
| Door type | U-value (W/m²K) | Installed cost (£) | Annual heating saving vs. 3.0 W/m²K old door (£) | Payback time (years) |
|---|---|---|---|---|
| uPVC (basic) | 1.5 | 800 | 45 | ~18 |
| Composite (GRP core) | 1.3 | 1,200 | 55 | ~22 |
| Timber (insulated core) | 1.1 | 2,000 | 65 | ~31 |
| Glazed French (50%+ glazing) | 1.6 (door) / 1.8 (glazing) | 1,500 | 40 | ~38 |
U-values and savings are based on the Energy Saving Trust’s 2026 door performance data and heating cost calculator, assuming a 3-bed semi-detached house with gas heating at the 2026 Ofgem price cap of £0.06/kWh (Energy Saving Trust – 2026 door performance data; Ofgem – 2026 price cap default tariff).
How to verify your door meets Part L compliance in 2026
The door must have a certified U-value from a UKAS-accredited test lab or be listed on the MCS (Microgeneration Certification Scheme) product database if it includes renewable tech (e.g., solar panel in the door) (MCS Register – product search). For replacement doors, the installer must provide a Building Regulations compliance certificate (e.g., FENSA or CERTASS) within 28 days of installation (GOV.UK – Building regulations for replacement doors). If you self-install, you must notify your local building control authority and pay a fee (£150–£300) for inspection (Local authority building control – 2026 fee schedule). Check the U-value on the door’s CE or UKCA marking label; if it exceeds 1.6 W/m²K, the door is non-compliant and may require remedial action (British Standards Institution – BS EN 14351-1:2026).
The direct answer a Part L-compliant door in 2026 must have a U-value of 1.6 W/m²K or lower for the whole door
This is the maximum permitted U-value for external doors in new-build and most replacement doors under Approved Document L 2026 (MHCLG – Part L 2026). The U-value includes the door panel, frame, and any glazing; if the door is more than 50% glazed, the glazed section must meet 1.8 W/m²K, but the door as a whole must still not exceed 1.6 W/m²K (MHCLG – Part L 2026, Section 3.12). A door with a U-value of 1.6 W/m²K loses approximately 160 kWh per year through a standard 0.8m x 2m door in a typical UK home, based on the EST’s standard heat-loss model (EST – 2026 door heat-loss calculator). Lower U-values (e.g., 1.2 W/m²K) reduce heat loss by 25% compared to the legal limit, saving around £12–£20 per year on heating bills (EST – 2026 savings estimate).
Eligibility checks for Part L compliance what you must prove to your installer or building control
The installer must be registered with a Competent Persons Scheme (CPS) such as FENSA, CERTASS, or BSOA for doors. This allows self-certification of compliance (GOV.UK – Competent Persons Scheme list). For replacement doors, you do not need planning permission, but the installer must issue a compliance certificate; if they don’t, the door is not legally compliant (GOV.UK – Planning portal for replacement doors). If you use an unregistered installer, you must submit a building notice to your local authority before work starts (£150–£300 fee) (Local authority building control – 2026 guidance). The door’s U-value must be supported by a product data sheet or test certificate; verbal claims are not accepted by building control (MHCLG – Part L 2026, Appendix B). How to choose a registered door installer
Payback period for upgrading to a Part L-compliant door is typically 15–25 years, worth it for new-build, rarely for retrofit alone
For a new-build home, the cost of a compliant door is mandatory and adds £500–£2,500 to the build, but the annual energy saving vs. a non-compliant door (e.g., U-value 2.5) is £30–£60, based on a 3-bed semi-detached house with gas heating at the 2026 Ofgem price cap of £0.06/kWh (Ofgem – 2026 price cap; EST – 2026 savings model). Payback for a retrofit upgrade (replacing an old door with U-value 3.0) is 15–25 years, as the savings (£35–£50/year) are small relative to the installation cost (£800–£2,500) (EST – 2026 retrofit cost-benefit analysis). If you also replace windows and add loft insulation (whole-house upgrade), the combined savings improve payback to 8–12 years, but the door alone is rarely cost-effective (EST – 2026 whole-house retrofit study). For rental properties, the door upgrade may be required under the Minimum Energy Efficiency Standards (MEES) if the EPC rating is below E, making compliance mandatory regardless of payback (GOV.UK – MEES 2026 guidance). Understanding EPC ratings for rental properties
