The denominator question
This is the most important point, and the most poorly handled. It concerns the area the benchmark relates to.
In a collective building several areas coexist: the sum of private areas, the total constructed area including circulation and common rooms, and the saleable or lettable area. The gap between them commonly reaches fifteen to twenty per cent.
A benchmark calculated on private areas alone is mechanically higher than one on total constructed area, since the numerator includes works the denominator ignores.
The gap is not marginal. Circulation, stair enclosures, plant rooms, cycle and refuse stores represent a real share of built area.
Before any collective benchmark the question is therefore: is this price related to private or to constructed area. The answer changes the result considerably.
The scale effect does not run as expected
In a house, increasing area slightly lowers the benchmark, because foundations and roof do not follow proportionally. The effect is regular and predictable there.
In collective housing that saving also exists, but it is offset by regulatory and technical thresholds adding cost above certain levels. Lift and fire safety thresholds are the clearest examples.
Crossing a threshold can therefore raise the benchmark even as area increases. This is counter-intuitive and the main source of error on this project type.
Items specific to collective housing
Six families of items have no equivalent in a house. They are costed separately and weigh on the benchmark per constructed area.
Common parts. Entrance halls, horizontal and vertical circulation, plant and service rooms. They are built, finished and maintained without being sold as habitable area.
Vertical transport. Above a certain number of storeys it becomes necessary and sometimes mandatory. It brings shaft, any machine room, maintenance and periodic inspection.
Acoustic requirements between dwellings. They impose heavier floor and wall build-ups than in a house, with structural consequences.
Fire safety. Compartmentation, smoke control, detection and escape follow requirements graduated by height and number of storeys.
Collective services. Individual metering, risers, ventilation, collective or individual heat generation depending on the concept adopted.
Parking. Local rules may impose it, and an underground solution radically changes the economics of the scheme.
| Item specific to collective housing | What it covers |
|---|---|
| Circulation and common parts | Entrance halls, stair cores, landings, bicycle and refuse rooms |
| Lift | Shaft, machinery, maintenance and mandatory inspections |
| Fire safety | Compartmentation, detection, smoke extraction and escape |
| Underground parking | Earthworks, shoring, waterproofing, ramp and ventilation |
| Collective services | Individual metering, rising mains, collective ventilation |
| External works within the plot | Internal roads, connections, planting |
The basement decides
On a dense scheme, underground parking is frequently the item that tips feasibility. Its cost per space far exceeds that of a surface space.
Earthworks, shoring, waterproofing, structure, access ramp and ventilation represent a high cost per space, of a different order from surface parking. The number of spaces imposed is therefore checked from the sketch stage.
That cost depends heavily on ground and water table, two unknowns removed only by geotechnical investigation. It also depends on the number of basement levels, each additional level costing more than the last.
What the region adds
Requirements applying to collective housing belong to each region, and they differ. They are checked with the competent regional administration.
Habitability standards, energy requirements and planning rules on density and massing are regional or even municipal. In Brussels the constraints of dense fabric are added, covered in the article on the Brussels region.
No collective benchmark is therefore transferable between regions without checking the applicable requirements.
What this means for an estimate
Four rules.
Name the denominator before any benchmark. Private or constructed, the gap is too large to remain implicit.
Check regulatory thresholds before fixing the number of storeys. One more level can trigger costly obligations.
Treat the basement as a project within the project, with its own investigation and its own decision.
Do not derive a collective benchmark from a house benchmark. The cost structures are not comparable.
This article reflects professional practice and regulation at the date of checking and serves as orientation. It does not constitute legal advice and does not replace assessment of the individual case.