Reference areas and how they differ

NBN B 06-002, published in June 1983 under the title "Areas and volumes of buildings, definitions and method of measurement", was the subject of a royal decree of 28 February 1984.

Its measurement principles are precise:

Rule	Content
Boundary	outer face of walls, centreline for party walls
Clear height	only floor areas with at least 1.50 m above them count
Service risers	included
Private staircases	counted at each level
Common parts	excluded

The 1.50 m rule is decisive for roof spaces. A room under a roof counts only for the part where that height is reached, which can remove a substantial share of a converted floor.

This point explains everything else.

Belgian standards form a reference system, but their application is voluntary. They become mandatory only where a regulation makes them so, and that has not been done for calculating habitable area.

Belgium differs here from France, where a precise calculation method is imposed on the sale of dwellings. The benchmarks of the two countries are therefore not directly comparable.

The consequence is that several methods coexist lawfully. A seller, an architect, a surveyor and an authority may each quote a different area for the same property without any of them being at fault.

Two notions circulate in everyday use, and the difference between them turns on two points only. They nonetheless produce appreciably different areas.

Gross area counts the living rooms including internal partitions, façade walls and half of party walls. It excludes cellars, unconverted lofts and common parts.

Net area corresponds to the area actually occupiable, after deducting risers and stair enclosures.

The difference therefore rests on two elements: where measurement starts, the inner or outer face of walls, and whether internal partitions are counted. Those two elements alone create a gap of several per cent.

In practice gross area is almost always quoted in Belgium, in property particulars as in construction offers. Net area is rarely communicated.

Walls and partitions represent a notable share of a dwelling's total area, in the order of fifteen per cent depending on configuration. A benchmark calculated on net area is therefore mechanically higher than one on gross area, at identical total price.

A third definition exists, this time regulatory, applying to Brussels projects. It serves the permit application and not the benchmark calculation.

The regional planning regulation uses the notion of floor area as defined by the regional regulation, defined as all floors excluding rooms with a clear height below 2.20 metres and rooms below ground level used as cellars, plant rooms or storage. Its height threshold differs from the one the standard uses.

Two differences from the standard deserve attention. They concern the height threshold and the excluded rooms.

The height threshold differs: 2.20 metres under the Brussels regulation against 1.50 metres under the measurement standard. A roof space may therefore count under one and not the other.

The exclusion criterion differs: the regulation reasons from the use of rooms, the standard from the nature of areas.

An area calculated for a Brussels permit application is therefore not directly transferable into a benchmark calculation, and vice versa. Each use calls for its own measurement method.

A regional variation sits on top of these definitions. Rules applying to dwellings, particularly requirements on clear height and natural light, belong to each region and differ.

In Brussels, clear height requirements apply to dwellings, with separate treatment for rooms under roofs. The other regions apply their own rules.

A habitable area calculated in Wallonia and one calculated in Brussels therefore do not necessarily cover the same thing, adding a further layer of incomparability to that already created by the absence of a mandatory standard.

Using the term "habitable area" without stating the method is not legally neutral. Professional liability has already been established on that basis.

Belgian case law has held a property professional liable for using that notion in a context where it could denote sometimes a gross and sometimes a net area. The court observed that the professional could simply have written "gross area".

The precaution is therefore elementary: name the convention. It costs three words and avoids a dispute.

Three rules for working properly with benchmarks in Belgium. They concern the method, its written statement and its consistency.

Never use a benchmark whose reference area is unnamed. Uncertainty in the denominator can exceed the gap between two offers.

Convert before comparing. Two benchmarks expressed on different areas must be brought to a common basis, which presupposes knowing both areas.

Keep the convention with the document. An in-house benchmark reused in two years without its measurement convention is unusable.