BHU Ventilation Requirements:
Option A vs Option B (Substitute)
Inadequate ventilation is the most common reason BHU applications are rejected. The Code of Practice offers two routes: meet the traditional natural ventilation standard (Option A), or use the substitute scheme (Option B) — a 0.1 sq m window plus mechanical ventilation at 5 air changes per hour. Understanding the difference is what prevents rejection.
| Option | Window Requirement | Mechanical Ventilation |
|---|---|---|
| Option A (natural) | Glass area ≥ 1/10 floor area Openable area ≥ 1/16 floor area |
Not required |
| Option B (substitute, common) | Glass area ≥ 0.1 sq m (light wells incl. enclosed ones accepted) |
≥ 5 ACH outdoor air + artificial lighting required |
| Bathroom (cl. 4.8.3) | Assessed separately | ≥ 5 ACH |
Source: BHU Code of Practice clauses 4.8.2–4.8.3 (Option B) · Option A technical standard: Buildings (Planning) Regulations ss.30–33
Option A: Meets Natural Ventilation Regulations
Option A applies to units where existing windows meet the traditional natural ventilation requirements under Buildings (Planning) Regulations s.30–33:
- Total window glass area: not less than 1/10 of the room's floor area
- Openable area: not less than 1/16 of the floor area
- Window orientation: must open directly to outdoor air (street, lane, courtyard or light well)
Units qualifying under Option A do not need a mechanical ventilation system — the lowest-cost route. In practice, most subdivided flats struggle to allocate enough external wall window area to each individual room to hit the 1/10 threshold, making Option B the more common compliance route.
Option B: The Substitute Route (Most Common for Subdivided Flats)
The BHU Code of Practice clause 4.8.2 sets out a substitute scheme specifically designed for spaces that cannot achieve Option A. Option B requires all three of the following simultaneously:
At least one openable window with glass area ≥ 0.1 sq m, providing a sufficient pathway to outdoor air (street, lane, courtyard or light well, including semi-enclosed and fully enclosed light wells)
A mechanical ventilation system supplying outdoor fresh air at a rate of at least 5 air changes per hour. An exhaust fan connected to an outdoor duct qualifies; an air-conditioning unit does not count.
The room must have artificial lighting installed to meet the minimum illumination requirement.
⚠️ All Three Option B Conditions Are Mandatory
Mechanical ventilation alone (without a qualifying window) does not satisfy Option B. Nor does a qualifying window alone. The most common error: a landlord installs an air-conditioning unit, assuming it counts as mechanical ventilation — it does not. Only a system supplying outdoor air qualifies.
Light Wells: Explicitly Accepted Under Option B
According to the BHU Code of Practice clause 4.8.2, Option B windows may open to any of the following:
- Street, lane, or passage
- Courtyard
- Light well — including semi-enclosed and fully enclosed (four-sided) light wells
Many older tenement subdivided flats have windows facing internal light wells — these are acceptable under Option B. The exception is a light well where a cover has been added at the top, potentially turning it into an enclosed space. Whether such a capped light well still constitutes a qualifying outdoor air pathway requires an on-site assessment by an Authorized Person.
⚠️ Real Case: Mechanical Ventilation System Rejected
A windowless subdivided unit in Sham Shui Po, renovated in 2021 and pursuing Option B, had a kitchen-style exhaust fan installed. On assessment, the airflow rate was well below the required 5 ACH, and the air-conditioning unit had been incorrectly counted towards the ventilation requirement. The system had to be redesigned with a compliant outdoor-ducted fan before certification was granted.
Bathroom Ventilation: Assessed Separately
Bathroom ventilation is governed by BHU Code of Practice clause 4.8.3 and assessed independently — it cannot share an air change allowance with the adjacent habitable room:
- Mechanical exhaust rate: at least 5 air changes per hour
- Exhaust duct must vent to the open air — not into common corridors or other rooms
- Bathroom ventilation is calculated independently, not shared with the room allowance
- A bathroom window with glass area < 0.1 sq m still requires supplementary mechanical ventilation
How to Fix a Ventilation Problem
Confirm qualifying window + install compliant mechanical system + add artificial lighting
Applies to most subdivided rooms with an existing window that doesn't meet Option A's 1/10 threshold. Confirm the window glass is ≥ 0.1 sq m and openable, then install an outdoor-ducted mechanical ventilation system providing ≥ 5 ACH. An Authorized Person designs and certifies the system; a licensed contractor installs it.
When the existing window cannot be used — add one opening to a light well
If the room's current window is fixed or faces a covered space, adding a small opening (≥ 0.1 sq m glass) towards an accessible light well may be feasible. The wall structure and any Deed of Mutual Covenant restrictions must be checked before works begin.
When a room is completely surrounded and cannot access any outdoor air pathway
If a room has no possible access to a street, courtyard or light well regardless of modifications, the only solution is to reconfigure the subdivision — typically by reducing the number of units so that each one has access to an external wall or light well.
Important: Mechanical ventilation systems require Buildings Department approval and an Authorized Person's written certification. Air-conditioning units do not qualify. Self-installed fans without AP oversight will be rejected.
Frequently Asked Questions
What are the two BHU ventilation options and what do they each require?
Under BHU Code of Practice clause 4.8.2, each habitable room must use either: Option A — natural ventilation meeting B(P)R s.30–33 (window glass ≥ 1/10 floor area, openable ≥ 1/16); or Option B (substitute) — openable window with glass ≥ 0.1 sq m opening to outdoor air (including enclosed light wells), plus mechanical ventilation at ≥ 5 ACH using outdoor air, plus artificial lighting. All three Option B conditions are mandatory.
Can a window facing a fully enclosed light well qualify for Option B?
Yes. The BHU Code of Practice clause 4.8.2 explicitly includes semi-enclosed and fully enclosed light wells as acceptable outdoor air pathways for Option B, provided the window glass area is ≥ 0.1 sq m and openable, combined with ≥ 5 ACH mechanical ventilation and artificial lighting. A light well with a roof added on top may not qualify — an Authorized Person must assess on-site.
Does an air-conditioning unit count as mechanical ventilation for Option B?
No. The Code of Practice specifies that the mechanical ventilation system must supply outdoor air — an air-conditioning unit only recirculates indoor air and does not meet the outdoor air supply requirement. Only systems with outdoor air intake and ductwork (such as exhaust fans connected to outdoor ducts) qualify.
Are bathroom ventilation requirements different from habitable rooms?
Yes. Under clause 4.8.3, bathrooms require a minimum of 5 air changes per hour. Bathroom ventilation is calculated independently — it cannot share an air change allowance with the adjacent room. The exhaust duct must vent to the open air, not into common corridors.
Does a mechanical ventilation system under Option B need Buildings Department approval?
Yes. The system must be designed by an Authorized Person to BD specifications, the design drawings submitted for Buildings Department approval, installed by a licensed contractor, and certified compliant by the AP in writing upon completion. Self-installed fans without this process will not be accepted for BHU certification.