How big does a lightwell need to be?

For natural daylight per Part L, the lightwell glazed area should be at least 5% of the room floor area it serves. So a 20m² basement room needs at least 1m² of lightwell opening (which would typically be 1m × 1m or larger). For escape windows (Part B), the lightwell must be large enough for a person to exit — minimum 850mm × 850mm clear internal dimensions at the base.

Does a lightwell need drainage?

Yes, all lightwells must have drainage to prevent water accumulating. Because lightwells are below ground level, a pump is needed to lift collected rainwater to the above-ground drainage system. A small submersible pump in a collection sump is the standard solution. The pump capacity should handle the worst-case rainfall — typically calculated using local rainfall intensity data.

Do I need a guardrail around a lightwell?

Yes, Approved Document K requires a barrier (guardrail) around any opening where there is a fall of 600mm or more. Lightwell guardrails must be at least 1100mm high and able to withstand a horizontal load of 0.36 kN/m for domestic situations. Options include glass balustrades, metal railings, or a structural cover/grating that can take pedestrian loads.

How do I waterproof a lightwell?

Lightwell walls should be waterproofed on the earth-retaining face with tanking slurry (Type A per BS 8102) or a cavity drain membrane continuation. The floor needs a waterproof membrane connecting to the basement waterproofing system. All junctions between lightwell and basement wall must be carefully detailed to prevent water tracking through the joint.

Can a lightwell serve as a fire escape?

Yes, a lightwell can provide an escape window route as an alternative to a second protected stairway. The window must have a minimum clear opening of 450mm × 450mm with at least 0.33m² area. The lightwell must have permanent steps or a fixed ladder to reach ground level, and the lightwell dimensions must allow a person to manoeuvre from the window to the escape route.

TradeCalculator

Lightwell Calculator

Calculate lightwell dimensions for Part L daylight, drainage pump requirements, guardrail lengths, and waterproofing quantities for basement lightwells.

Basement Floor Area

Floor area of room served by lightwell

Lightwell Length

Internal length of lightwell

Lightwell Width

Internal width of lightwell

Lightwell Depth

Depth from ground level to lightwell floor

Drainage Pump Required

Almost always required for below-ground lightwells

Guardrail Required

Required where fall >600mm (Part K)

How We Calculate This

This calculator determines lightwell dimensions and associated requirements for basement natural light, ventilation, and fire escape compliance per the Building Regulations.

Daylight calculation

Part L requires glazed areas equal to at least 5% of the floor area of the room served. The lightwell opening area should be at least this size to allow adequate daylight penetration to the basement window.

Drainage

A drainage pump is needed to remove rainwater that falls into the lightwell. The pump discharges to the above-ground surface water drain via a rising main with a non-return valve.

Waterproofing

Lightwell walls and floor are waterproofed using tanking slurry (1.5 kg/m² per coat × number of coats). The waterproofing must integrate with the main basement waterproofing system at the junction.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.