London HMO Conversion — Licensing, Fire Compartmentation, Article 4

Converting a family house (Use Class C3) into a House in Multiple Occupation (Use Class C4, small HMO ≤6 occupants; or sui generis large HMO ≥7) is a specialist London renovation category with two parallel consent regimes: planning and licensing. Permitted Development under GPDO 2015 Schedule 2 Part 3 Class L allows C3-to-C4 conversion without planning permission — but in the 30+ London boroughs with an Article 4 Direction removing Class L (Barking and Dagenham, Brent, Croydon, Ealing, Enfield, Haringey, Hounslow, Lewisham, Newham, Redbridge, Southwark, Waltham Forest, and others), full planning permission is required for any C3-to-C4 conversion.

The licensing regime is separate. Mandatory HMO licensing applies to any HMO with 5+ occupants forming 2+ households. Additional licensing (borough-specific) lowers the threshold — in some boroughs, any 3+ occupant HMO needs a licence. Selective licensing (also borough-specific) applies to all rented properties regardless of HMO status.

Building standards tighten significantly. Fire compartmentation to BS 9991 (residential) — 30- or 60-minute FD30/FD30S fire doors on every bedroom, self-closing devices, emergency lighting on the escape route, interlinked smoke/heat alarm system to BS 5839-6 Grade A or D1 LD1. Amenity standards (borough-specific): minimum bedroom size (usually 6.51 sq m single, 10.22 sq m double), shared kitchen and bathroom ratios, refuse storage, cycle parking.

AskBaily routes London HMO conversions to a specialist builder with active HMO licence experience in the target borough, BS 9991-compliant fire strategies, and Part L-compliant thermal upgrades.

HMO conversion compliance checklist

• Planning. Article 4 C3-to-C4 check in target borough. Full planning if Article 4 applies.
• Licensing. Mandatory HMO licence (5+ occupants) + additional licensing (borough-specific).
• Fire. BS 9991 strategy. FD30/FD30S doors, self-closing, BS 5839-6 Grade A or D1 LD1 alarms, emergency lighting, fire-risk assessment.
• Amenity. Minimum room sizes, bathroom/kitchen ratios, refuse, cycle storage.
• Thermal. Part L 2021 insulation + glazing.
• Electrics. Part P, 5-year EICR mandatory on HMOs.

Frequently asked questions

Do I need planning permission to convert a family house to an HMO in London? Depends on the borough. Article 4 Directions removing the Class L C3-to-C4 Permitted Development apply in 30+ London boroughs. Where Article 4 applies, full planning is required.

What HMO licence do I need? Mandatory HMO licensing applies to any 5+ occupant HMO of 2+ households. Additional licensing (borough-specific) lowers the threshold to 3+ in some boroughs. Selective licensing applies to all rented properties regardless of HMO status.

How much does an HMO conversion cost? £40,000-£180,000 for a typical 4-6 bedroom terrace-to-HMO conversion. Per bedroom: £8,000-£30,000 depending on fire-compartmentation scope and amenity upgrade.

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