Floor Area vs. Footprint: What Counts in Plot Ratio?

Two terms that sound almost identical but produce wildly different figures: gross floor area and building footprint. Confuse them, and you either over- or under-estimate the building rights — and both mistakes are costly. Estimate too low, and you leave square metres on the table and bid too cautiously; estimate too high, and you base your calculation on a volume the municipality will never approve.

The difference is not academic. The two areas feed into separate provisions that can each constrain a project independently of one another, and only one of them counts when the plot ratio is calculated. Here is the precise distinction between gross floor area and building footprint, including the grey zones around basements, attics and overhangs that cause the most uncertainty.

The two concepts: footprint vs. total floor area

The difference is easiest to remember geometrically.

• Building footprint is the building’s footprint on the site — the area within the outer faces of the external walls seen directly from above, regardless of how many storeys the building has. Two storeys stacked on top of each other give the same footprint as a single storey with the same floor plan.
• Gross floor area is the sum of every storey’s area, each one measured to the outer face of the external walls. It is the building’s total floor area, added up storey by storey.

A house with a 120 m² floor plan on two full storeys therefore has a footprint of 120 m² and a gross floor area of 240 m². It is the gross floor area — not the footprint — that feeds into the plot ratio:

Plot ratio = (total gross floor area ÷ site area) × 100

The footprint, by contrast, is governed by open-space requirements and distance to the boundary. The two figures work side by side, and a project can hit the ceiling on one long before the other: a low, sprawling building blows through the open-space requirement via its footprint, while a tall, narrow building hits the plot ratio via its gross floor area.

Gross floor area: what counts — and what doesn’t

This is by far the biggest source of error. Gross floor area is neither the footprint nor the total floor area of every room. It is a defined concept in the building code, where some areas must be included, others may be deducted, and a few don’t count at all. The starting point is that every storey’s area is measured to the outer face of the external walls and added together — but from there, three grey zones typically shift square metres back and forth.

Basement

Whether a basement counts towards the plot ratio is decided not by whether the room is called a basement, but by where the basement ceiling sits relative to ground level and what the room is used for. A basement whose ceiling lies below, or only slightly above, the surrounding terrain, and which is not fitted out for habitation or residence, can typically be kept out of the gross floor area. If, on the other hand, the basement is fitted out for residence — or raised so far that it effectively reads as a storey — it counts.

This means a partially buried “lower-ground” solution can shift an entire project from complying with the plot ratio to breaching it. The precise limit for how high the basement ceiling may sit is set in the current edition of the building code and should be verified there, or with the municipality, before it is used as the basis for a calculation.

Attic

For a converted attic, the area depends on the ceiling height. The area beneath the sloping walls counts only to the extent that there is sufficient height — the narrow strip right out at the eaves, where you cannot stand upright, is typically not included. That makes calculating a converted attic’s area a measurement, not an estimate: dormers, roof pitch and rafter construction determine how much of the loft area actually becomes countable gross floor area. Here it pays to calculate precisely rather than round, because an attic quickly contains square metres that either count in full or drop out entirely.

Carports, garages, sheds and covered areas

Secondary buildings and covered areas are treated separately. Carports, garages, sheds and open covered areas can be kept wholly or partly out of the gross floor area, within limits set in the building code. Overhangs and roof projections normally don’t count towards the footprint, because they aren’t part of the enclosed footprint — but an enclosed, heated extension does. It is precisely the line between “open covered area” and “enclosed room” where many calculations come apart.

The rules for all three grey zones can be supplemented or tightened in a local plan, and they are interpreted on a case-by-case basis by the municipality. Always verify them, therefore, in the current edition of the code and in the local plan, where building rights and use are set, before you lock in a figure.

Building footprint: why it constrains the project too

Although the plot ratio is calculated on gross floor area, the footprint is far from irrelevant. It is the footprint that decides whether the building can stand on the site at all within the distance requirements to the boundary, building lines and building zones — and it is the footprint that eats into the area that has to be left for open space, parking and amenity.

In practice, this means two projects with exactly the same gross floor area can have very different feasibility. Lay the gross floor area out on a single level, and the footprint becomes large, and you quickly hit the open-space requirement or the boundary distance. Stack the same area across several storeys, and the footprint shrinks, but then you instead run into the height-limit plane and the maximum building height. The maximum plot ratio tells you how much area you may build — not whether it can be placed on the site. That is determined by the footprint set against the site’s other provisions.

Where the two figures come from — and where they go wrong

Both areas can be looked up in public registers, but not without caveats. The existing gross floor area and footprint are found in BBR (the Buildings & Dwellings Register), the planning basis for what you may build in Plandata.dk, and the cadastral site area you calculate the ratio against in matriklen (the cadastre).

The pitfall is that BBR’s area concepts don’t necessarily map one-to-one to the building code’s gross-floor-area definition, and that registered areas aren’t always up to date. A converted attic or a converted basement may be recorded in BBR differently from how it actually is — and so give you a wrong starting point if you calculate residual capacity on the raw figure. Always check BBR against floor plans or a survey. It is one of several systematic sources of error covered in depth in the review of BBR data in property development and where it gets things wrong.

Once you have the two correct areas and the right site area, the building-rights calculation itself is mechanical. The whole calculation from cadastral parcel to a figure you can bid on is walked through step by step in the guide to how to calculate building rights and plot ratio on a cadastral parcel, and the difference between maximum and used building rights — the residual capacity — depends directly on the existing gross floor area being recorded correctly. That is how you find unused building rights on a cadastral parcel via residual capacity, once the area basis is in place.

From area concepts to a figure you can bid on

The manual exercise — deciding whether the basement counts, measuring the countable part of the attic, keeping covered areas and sheds out of the right concept, and cross-checking the existing area in BBR against the planning basis — is exactly the kind of detailed work where a single confusion of gross floor area and footprint shifts thousands of kroner in a calculation. That collation is what Arcili automates: in Ejendomme (Properties), BBR areas, planning status from Plandata.dk and the cadastral site area are gathered in one place, so you can read off both the footprint and the gross floor area — and see a property’s real volume — without searching across four registers.

It doesn’t replace the architect’s design work or the surveyor’s measurement — it saves you the hours in the early phase where you have to decide whether the areas add up at all before you bid. Want to see how gross floor area and footprint are read off a specific property? Book a walkthrough.