BGF, NRF and Wohnfläche: choosing the right reference area

The governing standard is DIN 277 in its August 2021 edition, which replaced the January 2016 edition of DIN 277-1 and now applies exclusively to building construction. It arranges the floor areas of a structure in a clear hierarchy.

The Brutto-Grundfläche (BGF), gross floor area, is the sum of all floor areas across every storey, measured to the external limits of the structure including the enclosing walls. It comprises two parts: It is the reference quantity of most published benchmarks.

BGF = NRF + KGF

The Konstruktions-Grundfläche (KGF), structural floor area, covers the footprint of the rising structure: external and internal walls, columns, and service and lift shafts up to a clear cross-section of 1.0 square metre. Shafts above that threshold count towards the net room area instead.

The Netto-Raumfläche (NRF), net room area, covers the usable floor areas and divides in turn into three parts. The distinction matters for conversion.

Abbreviation	German term	Content
NUF	Nutzungsfläche	Areas serving the actual purpose: dwelling, office, storage
TF	Technikfläche	Areas for building services plant such as heating and ventilation
VF	Verkehrsfläche	Circulation: corridors, stairs, entrance areas, lifts

The usage area categories run from NUF 1 to NUF 7. The 2021 edition added, under NUF 7, rooms whose intended use has not yet been fixed, which matters in early design stages.

The standard additionally distinguishes areas by how they are enclosed. The Regelfall (R), standard case, covers rooms fully enclosed on every boundary surface. The Sonderfall (S), special case, covers areas structurally connected to the building but not enclosed on every side, notably loggias, balconies, roof terraces, courtyards and external stairs. The suffix is placed in brackets after the area type, for example NUF (R) or NRF (S). In the special case, measurement runs to the limit of the vertical projection of the covering above.

For cost benchmarks the sensible reference is the complete gross floor area including both standard and special case, and that choice should be stated explicitly in every cost estimate. An above-ground gross floor area is occasionally used, for instance to assess a site's development potential against the permitted floor space index. For multi-storey buildings the difference from the complete gross floor area can be 20 per cent or more.

The Wohnfläche is not determined under DIN 277 but under the Wohnflächenverordnung, the habitable area ordinance. This is not a detail variation but a different counting method with a different purpose: the ordinance serves tenancy law and subsidised housing, DIN 277 serves design and cost planning.

Two differences carry the most weight in practice. First, Wohnfläche is measured clear of the structure, excluding wall cross-sections, whereas gross floor area is measured to external dimensions including every wall thickness. Second, whole categories of room fall outside it, in particular cellars, unconverted roof spaces, boiler rooms and garages, while areas under sloping ceilings count only in part. Balconies and terraces are included proportionally under the ordinance but held separately as a special case under DIN 277.

The consequence is clear. Anyone adding up habitable areas to infer the size of a building will land consistently too low.

Conversion runs through a factor stating how many square metres of gross floor area correspond to one square metre of habitable area. The orders of magnitude below are documented and usable for a sense-check, but do not replace measurement of the actual building.

Building type	Wohnfläche share of BGF	Factor: BGF per m² of Wohnfläche
Apartment building	around 53 to 56 per cent	about 1.80 to 1.90
Condominium unit in an apartment building	around 65 per cent	1.55 under Annex 24 of the German Valuation Act
Detached house with cellar and converted roof	lower depending on fit-out	as a rule well above 1.3
Detached house without cellar	around 70 per cent	about 1.4

The apartment building figure comes from an evaluation of 223 residential buildings and is therefore statistically robust. The factor of 1.55 is a normed value for condominium units and is used in tax valuation.

The frequently quoted claim that habitable area makes up 70 to 73 per cent of gross floor area applies to the favourable case of a detached house without a cellar and without elaborate circulation. In a house with a cellar and a converted roof space the share is clearly lower, because cellar areas enter the gross floor area in full but the habitable area not at all.

A benchmark of 2,100 euros per square metre of gross floor area is available. The building has 220 square metres of gross floor area and 150 square metres of habitable area, giving a factor of about 1.47.

• Against gross floor area: 220 × 2,100 = 462,000 euros
• The same amount against habitable area: 462,000 ÷ 150 = 3,080 euros per m² of Wohnfläche

The two figures, 2,100 and 3,080, describe the same building at the same price. Setting them side by side without conversion suggests a cost difference of 47 per cent that does not exist.

Alongside area, DIN 277 carries the Brutto-Rauminhalt (BRI), the gross volume of the structure. In practice cost benchmarks are held in parallel in euros per square metre of gross floor area and in euros per cubic metre of gross volume, and datasets frequently show both figures side by side.

The volume figure is more informative wherever storey heights depart sharply from the norm. A production hall with twelve metres clear height and an office storey with three metres have entirely different building costs at the same footprint, and the area benchmark does not capture that difference while the volume benchmark does. Conversely, in residential construction with largely uniform storey heights, the area benchmark is the more stable measure.

Practical rule: for buildings with standard storey heights use the area benchmark, and for halls and special structures cross-check against the volume benchmark. If the two results diverge sharply, one of the two quantities is wrong.

The area categories do not merely sit alongside one another; the ratios between them are a planning measure in their own right, describing how efficiently a design converts built area into usable area. Their ratios are themselves a measure of design quality.

Ratio	What it shows	Effect on cost
NUF to BGF	Share of purpose-serving area in the total	The lower it is, the more expensive each usable square metre becomes
VF to BGF	Share of circulation	Rises with finely divided uses and long corridors
KGF to BGF	Share of structure	Rises with small rooms and heavy construction
TF to BGF	Share of plant areas	Rises with the extent of building services, especially in non-residential work

This matters in cost planning because two designs with an identical benchmark per square metre of gross floor area can be very differently economic. The design with the higher usage area share delivers more workable floor space for the same money. A cost comparison that looks only at the building cost benchmark misses that difference entirely.

A robust options appraisal therefore carries the area schedule alongside the cost benchmark. Comparing only euros per square metre of gross floor area optimises the construction method rather than the project.

Purpose	Governing reference area
Cost framework and cost estimate	Gross floor area, supplemented by gross volume
Comparison of design options	Gross floor area together with the area schedule
Evidence under the German Buildings Energy Act	Nutzungsfläche
Development potential and floor space index	Geschossfläche under the land use ordinance
Letting and marketing	Wohnfläche or lettable area
Housing grant applications	Wohnfläche under the habitable area ordinance

The energy line is regularly overlooked: energy performance evidence works from the Nutzungsfläche, neither from gross floor area nor from habitable area. Anyone holding energy benchmarks and construction cost benchmarks in the same table already has two different denominators on the sheet.

Mixing benchmarks from different sources. Professional datasets work in euros per square metre of gross floor area, house builders and finance portals in euros per square metre of habitable area. A table combining both without conversion is worthless.

Treating Nutzfläche and Nutzungsfläche as the same thing. The former term Nutzfläche was replaced by Nutzungsfläche, and Netto-Grundfläche by Netto-Raumfläche. Older documents and older software still carry the earlier terms, sometimes with different boundaries.

Not recording the reference area. If a cost estimate carries only the number, it cannot be checked six months later, either by a third party or by its own author.

For a defensible cost statement, the complete gross floor area under DIN 277 is the reference, named explicitly and with a note on whether standard and special case were combined. Wohnfläche remains decisive for marketing, letting and grant applications, but does not serve as the basis for a cost comparison between projects.

Reference area is the first of four qualifiers that make a benchmark usable at all. The second concerns which cost groups sit inside the figure.