Subdivision of a flat (Subdivided units)

Subdivision of a flat into the commonly-known "subdivided units" generally refers to the subdivision of a flat as shown on the original approved plan of a building into two or more individual rooms.

The associated building works commonly involve the removal of the original non-structural partition walls, erection of new non-structural partition walls, installation of new toilets and kitchens, alteration or addition of internal drains, thickening of floor screeding to accommodate the new/diverted drain pipes, addition of door openings or ventilation openings, etc. The building safety and environmental hygiene may be adversely affected if these associated building works are not properly carried out, including addition of unauthorised door openings which contravene the regulations on means of escape, additions of openings for ventilation affecting the integrity of fire resisting construction, improper drainage works causing water seepage, and overloading of the building due to addition of non-structural partition walls and thickened floor screeding.

Building works associated with subdivision of a flat are subject to control under the Buildings Ordinance (BO). If the works involve the structure of a building, the owner should appoint an authorized person and a registered structural engineer, as may be required by the regulations, to prepare the alteration and addition plans. After obtaining approval of the plans and consent to the commencement of the works from the Building Authority (BA), the works should be carried out by a registered contractor in accordance with the approved plans.

If the works are designated minor works, they may be carried out through the simplified requirements under the Minor Works Control System (MWCS). The booklet "Minor Works Control System for interior renovation/alteration and subdivision of a flat" issued by the Buildings Department provides details of the minor works items associated with subdivision of a flat.

Building works (other than minor works) that are carried out in a building are exempted building works under section 41(3) of the Buildings Ordinance (BO) if they do not involve the structure of the building. For these exempted works, the Building Authority's (Buildings Department) prior approval of building plans and consent to the commencement of works are not required. Though not required to be carried out by prescribed registered contractors and supervised by prescribed building professionals, the works should not be carried out in contravention of the regulations (which specify the building design and construction standards in such aspects as fire safety, structural safety and drainage works) under the BO.

Owners who intend to carry out addition and alteration works to buildings should consult building professionals on feasibility of the works, whether the works are minor works or works that require BA's prior approval and consent, to ensure that the works do not contravene the BO.

The building works associated with subdivision of a flat commonly involve the removal of the original non-structural partition walls, erection of new non-structural partition walls, thickening of floor screeding and addition of door openings. These works may overload the floor slabs, beams and even the columns of the building. The structure of the building would be adversely affected due to the addition of openings on structural walls for new doors or for new drain pipes installation. If the relevant drainage works are not properly carried out leading to water seepage, corrosion of steel reinforcement and concrete spalling at the ceiling of the flat below may occur.

Building works associated with "subdivided units" commonly involve installation of new toilets/kitchens, alteration or addition of internal drainage system for the new toilets/kitchens. If the works are not properly carried out, water seepage may occur and the environmental hygiene of the building may be adversely affected. Moreover, windows of adequate areas for natural lighting and ventilation should be provided to every habitable room, bathroom, lavatory or kitchen in order to comply with the requirements stipulated under the Building (Planning) Regulations, and construction of new toilet/kitchen on any balcony built over the street is not permitted.

Change in Use of Flat

1. Do not subdivide flats in non-domestic buildings especially industrial buildings into individual rooms for dwelling purposes.

Fire Safety

1. Do not remove existing fire rated doors or replace them by doors not meeting the required fire resistance rating.
2. If the fire rated door at the original entrance of the flat is maintained and the total number of occupants of the flat is not more than 30 persons, the clear width of the internal corridor serving the "subdivided units" should not be less than 0.75m. In the case of more than 30 persons but not more than 200 persons, the clear width of the internal corridor should not be less than 0.85m. (see Diagram 1)
3. If the fire rated door at the original entrance of the flat is removed or replaced by a door not meeting the required fire resistance rating, the clear width of the internal corridor serving the "subdivided units" should not be less than 1.05m. Besides, the partition walls between the "subdivided units", the walls at both sides of the internal corridor and the door of each "subdivided unit" should have a fire resistance rating of not less than 1 hour. Such doors should also be self-closing and smoke-sealed. (see Diagram 2)
4. Exit routes should have a clear headroom of not less than 2m.
5. Do not make new door openings or ventilation openings in the enclosures of exit staircases.
6. Do not make openings for ventilation or for installation of air-conditioners, exhaust fans, etc. in the fire rated walls separating the units and the means of escape.
7. Metal gates, if installed, should not obstruct the means of escape.
8. "Subdivided units" should not block or obstruct the means of escape leading to any exit staircase. (see Diagram 3)

Building Structure

1. Do not overload the building structure due to erection of non-load bearing block walls and thickening of floor slabs by laying solid screeding.
2. The simplified requirements of the MWCS for Class I minor works may be followed for erection of any non-load bearing block wall, laying of any soild floor screeding, or erection or alteration of any aboveground drain in a domestic flat that is subdivided into 3 or more rooms each provided with lavatories or other sanitary fitments and the resulting number of such rooms is greater than that shown on the original approved plan.
3. The simplified requirements of the MWCS for Class III minor works may be followed for erection of any non-load bearing block wall in a domestic flat. The density of the wall is not more than 650 kg per m³; its height is not more than 3 m; its thickness is not more than 75 mm; and the aggregate length of any additional wall per m² of the floor area of the flat is more than 0.1 m but not more than 0.3 m. Moreover, the thickness of the floor slab supporting the wall is not less than 125 mm and the thickness of the floor screeding is not more than 25 mm (measured from the structural floor level). The works should not result in any additional load to any cantilevered slab and should not involve alteration of any other structural elements.

4. The simplified requirements of the MWCS for Class III minor works may be followed for thickening of any floor slab of a domestic flat, which is not less than 125 mm thick, by laying solid screeding. If the density of the screeding is not more than 650 kg per m³, its thickness could be more than 25 mm but should not more than 75 mm (measured from the structural floor level). If the aggregate area of the screeding is not more than 1.5 m² within every floor area of 10 m² of the flat and the nearest horizontal distance between each of the area of screeding is not less than 2 m, the thickness of the screeding could be more than 25 mm but not more than 150 mm. Moreover, the works should not result in any additional load to any cantilevered slab.

   If the density of the screeding is more than 650 kg per m³ but not more than 1200 kg per m³, its thickness could be more than 25 mm but should not more than 45 mm (measured from the structural floor level). If the aggregate area of the screeding is not more than 1.5 m² within every floor area of 10 m² of the flat and the nearest horizontal distance between each of the area of screeding is not less than 2 m, the thickness of the screeding could be more than 25 mm but not more than 100 mm. Moreover, the works should not result in any additional load to any cantilevered slab.

   If any non-load bearing block wall is erected in the flat, the aggregate length of any additional wall per m² of the floor area of the flat is less than 0.1 m, and the conditions on density, height and thickness of the additional wall, and on the thickness of the floor slab supporting the wall as specified above of the Class III minor works for erection of non-loading bearing block wall should be complied with.

5. Do not make openings on structural walls, columns, beams or slabs of buildings unless prior approval and consent have been obtained from the BA or such works are to be carried out under the simplified requirements of the MWCS.
6. Drainage works should be properly carried out to avoid leakage which may lead to corrosion of steel reinforcement bars undermining the structure of the building.

Environmental Hygiene

1. Unless exempted by the BA, any part of a domestic building which is intended to be separately let for dwelling purposes should be provided with kitchen accommodation.
2. The simplified requirements of the MWCS for Class II and Class III minor works may be followed for erection, alteration or removal of any aboveground drain.
3. Installation of drains and sanitary fitments should meet the requirements stipulated in the Building (Standards of Sanitary Fitments, Plumbing, Drainage Works and Latrines) Regulations.
4. To avoid water seepage to the floor below, apart from ensuring quality workmanship for the installation of drains and sanitary fitments, the floor of every room with water supply should also be constructed with impermeable materials.
5. Construction of a toilet/kitchen on any balcony built over streets is not permitted.
6. Headroom for areas for habitation should not be less than 2.5m.

Lighting and Ventilation

1. Windows of adequate areas for natural lighting and ventilation should be provided to every habitable room, bathroom, lavatory or kitchen in order to comply with the requirements stipulated under the Building (Planning) Regulations.
2. If adequate windows cannot be provided to internal bathrooms or lavatories, a system of artificial lighting and mechanical ventilation producing 5 air changes per hour should be in operation at any time when the bathroom or lavatory is in use. The change of air shall be with the outside of the building and to achieve this, the use of ducting for ventilation provided with access for cleaning is acceptable.

If the following conditions are observed, the building works associated with subdivision of a flat may have irregularities:

1. Subdivision of non-domestic buildings especially industrial buildings into individual rooms for dwelling purposes
2. The means of escape is obstructed by doors/metal gates installed at the exit of "subdivided units".
3. The exit staircase of the building is not accessible from the "subdivided units".
4. Door/ventilation openings are formed in the enclosure of the exit staircase.
5. Structural members (e.g. beams, columns or structural walls) are partially removed.
6. Openings are formed on structural walls.
7. Toilet and kitchen are constructed on the balcony built over street.
8. No window is provided in the "subdivided units" for natural lighting and ventilation.
9. There are signs of leakage of the new/diverted drain pipes.
10. There are signs of water seepage or concrete spalling at the ceiling of the flat below.
11. Cracks appear at the junction of the partition wall and ceiling.
12. Cracks appear on the floor near the cantilevered balcony that may caused by overloading.

If you are the owner of such premises and suspect or find out there are irregularities of the building works associated with subdivision of a flat, e.g. blocking of the access to the exit staircase, removal of structural members, water seepage or concrete spalling, you should immediately consult a building professional and appoint a registered contractor to rectify the irregularities so as to protect yourself and other tenants. You should also report the situation to the Buildings Department.

If you are the tenant of a "subdivided unit" or intend to rent a "subdivided unit" and discover the above-mentioned situation which may contravene the Buildings Ordinance, you should immediately ask the owner to carry out rectification works and report the case to the Buildings Department through: