The Simplest Route to Part L Compliance

Extension Reference Method

Keep glazing under 25% of floor area and meet standard U-values. No SAP calculation needed - Building Control can sign off on specification alone.

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Extension Reference Method

What is the reference method?

The simplest Part L compliance route. Meet two conditions and no energy calculation is needed.

Learn more →

What is the 25% rule?

Total glazing must be ≤25% of extension floor area, plus any existing openings you cover over.

25% rule explained →

What U-values do I need?

Walls 0.28, floors 0.22, flat roofs 0.18, pitched roofs 0.16, windows 1.6 W/m²K.

See U-value limits →

How do I calculate my allowance?

Floor area × 0.25 + existing openings being covered = your maximum glazing.

Calculate yours →

What if I exceed 25%?

Try area-weighted method next - better insulation elsewhere can compensate.

Area-weighted method →

Can I check this myself?

Yes. Use our free calculator for instant pass/fail and a PDF for Building Control.

Open calculator →

Check if your extension passes the reference method in 60 seconds with our free calculator.

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What Is the Reference Method?

The reference method is the simplest route to Part L compliance for extensions. As described in section 10.7 of Approved Document L, if your extension meets two conditions, Building Control can approve it without any energy calculation.

This is how most rear and side extensions are approved. Standard double glazing, reasonable insulation, and modest window sizes typically pass without issue. No SAP assessor, no calculation fees, no waiting for reports.

The two conditions are:

  1. Glazing area - total windows, doors and rooflights must be ≤25% of extension floor area, plus any existing openings you're covering over
  2. U-values - walls, floor, roof and openings must meet Part L limiting standards

The 25% Glazing Rule Explained

Part L allows glazing (windows, doors and rooflights combined) up to 25% of your extension's floor area. But there's a bonus most people miss - you can also add the area of any existing openings you're covering over.

Example Calculation

A 20m² rear extension removing a back door (2m²) and kitchen window (1.5m²):

  • Base allowance: 20m² × 0.25 = 5m²
  • Plus existing openings: 2m² + 1.5m² = 3.5m²
  • Total glazing allowance: 8.5m²

This is often more generous than people expect. A 4m wide bifold door is typically around 6-7m², well within this allowance for most extensions.

U-Value Limits for Extensions

To pass the reference method, all thermal elements must meet these Part L limiting U-values:

Walls

  • ✓ External walls: 0.28 W/m²K
  • ✓ Party walls to unheated spaces: 0.28 W/m²K
  • ✓ Standard cavity wall with 100mm insulation typically achieves this

Floors

  • ✓ Ground floors: 0.22 W/m²K
  • ✓ 100mm PIR insulation below screed usually sufficient
  • ✓ Suspended floors need similar specification

Roofs

  • ✓ Flat roofs: 0.18 W/m²K
  • ✓ Pitched roofs: 0.16 W/m²K
  • ✓ 150mm+ PIR for flat, 300mm mineral wool for pitched

Openings

  • ✓ Windows: 1.6 W/m²K
  • ✓ Doors: 1.6 W/m²K (half glazed)
  • ✓ Rooflights: 2.2 W/m²K

Most standard modern specifications meet these limits. If your builder or architect has specified reasonable insulation levels, you're likely fine.

Calculate Your Glazing Allowance

The formula is simple:

(Floor Area × 0.25) + Existing Openings = Maximum Glazing

Count all glazed elements in the extension:

  • Windows (use frame outer dimensions)
  • Glazed doors including bifolds and sliding doors
  • Rooflights and roof lanterns
  • Any glazed panels in walls

If your total glazing is within the allowance and U-values meet limits, you pass. No calculation needed - your builder can submit the specification to Building Control directly.

Check Your Extension in 60 Seconds

Enter your dimensions and get an instant pass/fail result. Download a PDF summary for Building Control.

Free Glazing Calculator →

What If Your Glazing Exceeds 25%?

Exceeding the reference method limit isn't the end. Part L provides two more routes to compliance - most extensions still pass using one of these.

Area-Weighted U-Values

Better insulation elsewhere can compensate for extra glazing. £95 check, credited toward SAP if needed.

Area-Weighted Method →

Whole House SAP

Models the entire dwelling. Solar gains and heating efficiency often approve heavily glazed designs.

Whole House SAP →

Reference Method FAQ

Do I need to submit anything to Building Control?
Just your specification showing U-values and glazing areas. No formal calculation or report is needed if you pass the reference method.
What counts as an existing opening?
Any window, door or other glazed opening in the existing external wall that will be within the new extension. The opening must be being covered by the extension works.
Do conservatory doors count?
If your extension connects through where a conservatory door was, that door area can be added to your allowance (assuming the conservatory is being removed or incorporated).
What about roof lanterns?
Roof lanterns count as glazing and must be included in your 25% calculation. They also need to meet the 2.2 W/m²K rooflight U-value limit.
Can I use the reference method for a conservatory?
If the conservatory is thermally separated from the house (external doors between, independent heating), different rules apply. If open to the house, treat it as a normal extension.

Check your extension in 60 seconds

Free Glazing Calculator → Over 25%? Get SAP Quote

Most extensions pass the reference method. Check yours for free.