Determination of full-area vs half-area gross floor area: the four space-category rules of GB 55031-2022 Section 3.1, special parts (balconies / sloped roofs / basements) and the six categories not counted

Topic Positioning

Determination method for gross floor area "full area / half area (1/2 area) / not counted". The current governing basis is Section 3.1 of GB 55031-2022 "General Code for Civil Buildings" (a fully mandatory engineering-construction code), with GB/T 50353-2013 "Code for Calculation of Building Area" providing supplementary detailed rules for reference. Applicable to area-basis determination in building design, construction-permit application, planning verification, construction-drawing budgeting and property surveying.

Core Conclusions

1. Preconditions for counting (GB 55031-2022 Clause 3.1.4): only building spaces of permanent structure, having a permanent top cover and a structural storey height or sloped structural slab-top height of 2.20 m or above, are counted in gross floor area. None of the three preconditions may be missing: temporary, mobile and simple buildings are excluded; temporarily erected top covers of any kind (e.g. temporary sunshades) are excluded; and building spaces with a storey height or slab-top height below 2.20 m are not counted (Clause 3.1.6).

2. Four full/half determination rules (GB 55031-2022 Clause 3.1.4):

1. Building space with an envelope structure and closed enclosure - full area: calculated as the horizontal projection of the space bounded by the outer surface of its external envelope structure;
2. Building space without an envelope structure, bounded by columns, or not closed and enclosed jointly by envelope structure and columns - full area: calculated as the horizontal projection of the space bounded by the columns or by the outer surface of the external envelope structure;
3. Building space without an envelope structure, with a single row of columns or freestanding columns, not closed - 1/2: calculated as 1/2 of the horizontal projection area of its top cover;
4. Building space without an envelope structure and without columns but provided with enclosure facilities attached to the external envelope structure of the building, not closed - 1/2: calculated as 1/2 of the horizontal projection of the space bounded by the outer surface of the enclosure facilities.

Memory aid: enclosed = full; column-bounded = full; single-row/freestanding column = half; column-free with enclosure facilities = half. The typical spaces matched to each item in the code commentary - Item 1: all usable spaces within the external envelope structure and partial storeys; basements, semi-basements and their corresponding entrances; deformation joints connected with the interior; equipment floors, pipe floors and refuge floors; tiered book depositories/warehouses/garages; tank rooms and elevator machine rooms; lobbies, halls and their galleries; enclosed corridors, overhanging corridors and connecting corridors; roofed light wells, etc. (excluding non-accessible indoor top covers in exhibition buildings, airports etc. carrying uniformly distributed loads not exceeding 0.5 kN/m2 and point loads not exceeding 1 kN); Item 2: canopies, car sheds, goods sheds and platforms enclosed by walls and columns, roofed platforms, roofed sky gardens, porches, entrance vestibules, outdoor stairs, underground garage entrances, columned outdoor connecting corridors, stilt (open ground) floors, etc.; Item 3: outdoor connecting corridors, car sheds, goods sheds and platforms supported by a single row of columns or freestanding columns, etc.; Item 4: column-free outdoor overhanging corridors, connecting corridors and eaves galleries, overhanging column-free outdoor stairs, etc. (column-free canopies not included).

3. Calculation interface (Clause 3.1.1): gross floor area shall be calculated, at the floor (ground) level of each natural storey, as the horizontal projection of the space bounded by the outer surface of the external envelope structure. "At the floor (ground) level" means the finished design surface; for ground without a structural floor slab it is the top of the concrete blinding; the "external envelope structure" includes glass curtain walls, metal curtain walls, stone curtain walls and artificial-panel curtain walls; the "outer surface" is the finished design surface, i.e. the outer edge of the finish layers (including thermal insulation and finish); where a curtain wall serves as the external envelope, it is the outer edge of the curtain-wall panel.

4. Balconies (Clause 3.1.5): the gross floor area of a balcony shall be calculated as 1/2 of the horizontal projection of the space bounded by the outer surface of its enclosure facilities; where the balcony is enclosed, it shall be calculated as the horizontal projection of the space bounded by the outer surface of its external envelope structure (full area).

5. Six categories not counted (Clause 3.1.6): (1) building spaces with a structural storey height or sloped structural slab-top height below 2.20 m; (2) building spaces without a top cover (outdoor platforms, turrets and terraces, outdoor steps and ramps, building roofs, roof gardens, pergolas, unroofed elevated open corridors, working platforms, material-loading platforms, equipment platforms, etc.); (3) components (fittings) attached to the external envelope structure of the building (decorative columns, door and window moldings, plinths, air-conditioner slabs, sunshade panels, building eaves, column-free canopies, etc.); (4) the space beneath overhanging parts of a building; (5) public traffic spaces within a building used as urban streets and lanes (qilou arcades, over-street passages, etc.); (6) structures of all kinds independent of the building (chimneys, water towers, water (oil) tanks, trestles, silos, oil (water) storage ponds, etc.).

6. Above-ground / below-ground division (Clause 3.1.3): building spaces above the outdoor design ground level count into above-ground gross floor area, those below into below-ground; where the ground level falls within a building space, the space counts as below-ground when the height of its floor below the ground level exceeds 1/2 of the storey height, and as above-ground when it is less than or equal to 1/2; sloping sites are calculated at the average height.

7. Supplementary detailed rules of GB/T 50353-2013 (recommended, for reference):

• Clause 3.0.1: spaces with a structural storey height of 2.20 m or above count in full; below 2.20 m count 1/2 (note the basis difference from GB 55031-2022, see Applicable Boundaries);
• Clause 3.0.3 sloped roofs: parts with a structural clear height of 2.10 m or above count in full; parts of 1.20 m or above but below 2.10 m count 1/2; parts below 1.20 m are not counted;
• Clause 3.0.5 basements and semi-basements: calculated by the horizontal area within the structure; structural storey height >= 2.20 m counts in full, < 2.20 m counts 1/2;
• Clause 3.0.21 balconies: balconies within the main structure count in full by the horizontal area within the structure; balconies outside the main structure count 1/2 by the horizontal projection of the structural floor slab.

Applicable Boundaries

• GB 55031-2022 is a fully mandatory engineering-construction code (in force from 1 March 2023); gross-floor-area calculation of civil buildings must follow it. GB/T 50353-2013 is a recommended standard providing detailed rules for reference. Where the two disagree on the same situation, the mandatory code prevails - typical differences: (1) spaces with storey height below 2.20 m: GB/T 50353-2013 Clause 3.0.1 counts 1/2, whereas GB 55031-2022 Clause 3.1.6 does not count them; (2) balconies: GB/T 50353-2013 counts full/half respectively inside/outside the main structure, whereas GB 55031-2022 uniformly counts 1/2 by the outer surface of the enclosure facilities, with enclosed balconies in full.
• The area basis varies by purpose (permit application, planning verification, construction-drawing budget, property surveying); the same part may adopt different bases at different stages - clarify the basis before use; where local standards or detailed rules exist for the project location (e.g. "multi-survey integration" and local property-surveying standards refining the pre-calculation scope), those provisions govern.
• This card is mainly on the civil-building basis and does not replace the project-location implementation documents or the basis agreed in the contract.

Further Reading

• GB 55031-2022 "General Code for Civil Buildings", Clauses 3.1.1, 3.1.3, 3.1.4, 3.1.5 and 3.1.6 and their commentaries (including the lists of applicable spaces for each item and Figures 1-4).
• GB/T 50353-2013 "Code for Calculation of Building Area", Clauses 3.0.1 (storey-height full/half), 3.0.3 (sloped-roof clear-height banding), 3.0.5 (basements and semi-basements), 3.0.21 (balconies).
• Example of local refinement: DB11/T 2032-2022 "Technical Specification for Multi-Survey Integration of Engineering Construction Projects", Clause 6.2.6 (full/half/non-counting scope lists for pre-calculation of building area and the shared-area apportionment formula).
• Standard-history note: the current recommended standard for building-area calculation remains GB/T 50353-2013 (there is no GB/T 50353-2023); upon implementation of GB 55031-2022 the mandatory provisions of 9 then-current standards were abolished - its fully mandatory status can be verified in the announcement of the Ministry of Housing and Urban-Rural Development.