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Lightwell Calculator
Size a basement lightwell for daylight, drainage pump requirements, guardrail lengths and waterproofing quantities — using the ADL1A 1/20 daylight rule of thumb and Approved Document K guarding.
Floor area of room served — used for the 1/20 daylight rule of thumb (ADL1A guidance, not a Part L minimum)
Internal length of lightwell
Internal width of lightwell
Depth from ground level to lightwell floor
Almost always required for below-ground lightwells
Required where fall >600mm (Part K)
Tanking slurry coats (2-3 typical) — quantity uses ≈1.5 kg/m²/coat; confirm against your product PDS
Your quoted guardrail supply + fix rate
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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 check (guidance, not a Part L rule)
Part L imposes no minimum daylight requirement — Approved Document L1A 2010, clause 4.27, states that “no minimum daylight requirement is specified.” As a rule of thumb from that guidance, glazing much less than 1/20 (5%) of the room floor area gives poor natural light, so this calculator flags whether the lightwell opening reaches that 1/20 lower bound. It is advisory only, not a compliance threshold; many habitable rooms are designed nearer 1/10 (England general guide) or 1/15 (Scottish Technical Handbook standard 3.16). For detailed daylight design, BS EN 17037:2018 (which superseded the withdrawn BS 8206-2) sets out daylight-factor and illuminance methods.
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 cementitious tanking slurry. The quantity here uses an indicative ≈1.5 kg/m² per coat × number of coats — always confirm against your chosen product’s data sheet, as rates vary (for example Newton HydroCoat ≈2.8 kg/m² over two coats, Feb Febtank Super ≈3.5 kg/m² over two coats). The waterproofing must integrate with the main basement waterproofing system at the junction.
Excavation volume
Excavation is the lightwell void × an indicative ×1.2 working-space allowance for formwork, wall thickness and access. This is a trade rule of thumb, not a fixed figure — actual over-excavation depends on the retaining-wall construction, any shoring or batter, and site access.
Frequently Asked Questions
Part L sets no minimum daylight requirement (Approved Document L1A 2010, clause 4.27 states 'no minimum daylight requirement is specified'). As a rule of thumb from that same guidance, glazing much less than 1/20 (5%) of the room floor area gives poor natural light, so the lightwell opening is often sized to give the basement window at least this area — a 20m² room would suggest about 1m² of opening (around 1m × 1m or larger). Many designers aim higher (roughly 1/10 in England, or 1/15 under Scottish Technical Handbook 3.16) for habitable rooms, so treat 1/20 as a lower bound rather than a target. Where the lightwell window also serves as a fire-escape window, Approved Document B governs its size (see the fire-escape FAQ below).
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.
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.
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.
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.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.