Reference Method Compliance Check for Extensions
The simplest method for extensions within the opening allowance: 25% of floor area plus covered existing windows and doors. The fabric and services must also meet the applicable standards.
- The cheapest and quickest compliance route
- For openings within 25% of floor area plus covered existing windows and doors
- No full SAP Calculation needed
- Accepted by Building Control across the UK
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What Is the Reference Method?
The Reference Method is the simplest of the three routes to Part L compliance for an extension, and is set out in section 10.7 of Approved Document L, Volume 1 (Dwellings). It checks the opening allowance and the fabric specification. The allowance is 25% of the extension floor area plus existing windows and doors covered or removed because of it. The walls, roof, floor and openings also need to meet the applicable standards.
Meet both and your extension passes. There is no full SAP Calculation, no lengthy modelling and no waiting. Most standard rear and side extensions with typical window sizes qualify, which is why it is the cheapest route for the majority of projects.
The 25% Glazing Rule Explained
Part L allows glazing (all your windows, glazed doors and rooflights added together) of up to 25% of the extension floor area. There is also a bonus most people miss: you can add the area of any existing openings you are covering over.
Worked example
A 20m² rear extension that removes an existing back door (2m²) and kitchen window (1.5m²):
- Base allowance: 20m² × 0.25 = 5m²
- Plus the openings being covered: 2m² + 1.5m² = 3.5m²
- Total glazing allowance: 8.5m²
That is more generous than it first looks. A 4m set of bifold doors is usually around 6m² to 7m², which fits comfortably within the allowance on most extensions. Check your own glazing free to see where you stand.
What Counts Towards Your Glazing
Add up every glazed element in the extension, using the outer frame dimensions:
- Windows
- Glazed doors, including bifold and sliding doors
- Rooflights and roof lanterns
- Any glazed panels in the walls
The formula is simply:
(Floor area × 0.25) + existing openings covered = your maximum glazing
If your total glazed area is within that figure and your U-values meet the limits below, you pass. Your builder can submit the specification to Building Control directly, with no calculation required.
U-Value Limits for Extensions
Every new thermal element in the extension must meet the maximum U-values in Table 4.2 of Approved Document L, Volume 1 (Dwellings), which Section 10 applies to extensions:
- Walls: 0.18 W/m²K
- Floors: 0.18 W/m²K
- Roof: 0.15 W/m²K
- Windows: 1.4 W/m²K
- Roof windows: 1.4 W/m²K
- Doors: 1.4 W/m²K
- Rooflights: 2.2 W/m²K
Most modern specifications meet these comfortably. For the full tables and common build-ups that pass, see our guide to Building Regulations U-values.
When the Reference Method Works
The Reference Method is the right route when:
- Your openings are within 25% of floor area plus covered existing windows and doors
- It is a standard single or double storey rear or side extension
- All elements meet the limiting U-values
- You want the quickest and cheapest way to satisfy Building Control
- You are not adding large bifolds, picture windows or multiple rooflights
What If You Exceed the Opening Allowance?
If your openings exceed the full allowance, including covered existing windows and doors, two further methods can demonstrate compliance.
Pricing
Need Area Weighted Instead?
£125
If your openings exceed the full allowance, an Area Weighted U-value calculation may demonstrate compliance.
As a non-VAT-registered business, SAPgen does not charge VAT. All prices shown are the total payable.
Frequently Asked Questions
What if my extension has a lot of glazing?
What counts as an existing opening?
Do roof lanterns and rooflights count?
Do I need to submit a calculation to Building Control?
What information do you need?
What if it fails?
Check Your Glazing Allowance Free
Use our free calculator to check your extension opening allowance.