How to Calculate Building Rights and Plot Ratio on a Plot

Building rights determine how many square metres a site can carry — and therefore what it is actually worth. A bid on a building site is really a bid on the floor area the site can accommodate, and the price per square metre of building right is the figure that decides whether the deal stacks up. Yet the plot ratio is often miscalculated, because it is unclear what counts and what the site area really is.

Calculating building rights and plot ratio correctly is therefore not a formality — it is the foundation under the entire calculation. A ten percent error in the floor area feeds straight through to the residual value of the site and can be the difference between a project that holds and one that does not. This guide takes you all the way from the cadastral parcel to a figure you can keep building on, and points out along the way the places where the calculation typically goes off the rails.

The basics: what is plot ratio?

The plot ratio expresses the total floor area as a percentage of the site area. If you build 400 m² of floor area on a 1,000 m² site, the plot ratio is 40. The formula is simple:

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

The simplicity of the formula hides the fact that both variables can be interpreted — and that is precisely where the errors arise. If you count the wrong areas into the floor area, or calculate from a site area that is in reality smaller than the cadastre shows, you get a figure that will not survive a building permit review. The building right on a parcel is the result of the correct floor area set against the correct site area — not a quick estimate.

The maximum plot ratio is determined in this order: an applicable local plan takes precedence, then the framework provisions of the municipal plan, and if neither says anything, you fall back on the guidance values in the building code — typically 30% for detached low-density, 40% for dense low-density and 60% for apartment buildings. Always find the real ceiling in the local plan, where building rights and use are set before you calculate further.

Step 1: establish the correct site area

The first thing many people take for granted is the site area — and that is a mistake. The cadastral area shown in the cadastre (matriklen) and in the title section of the Land Registry (tingbogen) is the starting point, but it is rarely the area you may calculate your building rights from.

Several things can reduce the basis for the calculation:

• Road area. If the site must cede area to a designated road or path, that area generally does not count towards the basis for the plot ratio.
• Areas under specific area types. Lakes, watercourses and protected areas may be exempt, depending on the planning framework.
• Shared open space and subdivision. If the site is to be subdivided and area is set aside for shared access or open space, that area is deducted from what each new parcel may calculate from.

Always look up the cadastral area in the cadastre (matriklen) and cross-check it against what the local plan says about the basis for the calculation. Two sites that are equal on paper can have markedly different building rights if one must cede land to a road and the other does not.

Step 2: find what counts towards the floor area

This is where the largest source of error lies. The floor area is not the same as the footprint — the building’s imprint on the site — and it is not the total floor space of every room either. It is a defined concept in which certain areas must be counted in, others may be deducted, and a few do not count at all.

The floor area is measured under the building code, as a starting point, to the outer face of the external walls and covers the area of all storeys. The places where interpretation moves square metres:

• Basement. A basement whose ceiling sits less than a certain height above ground level, and which cannot be used for habitation, can typically be kept outside the floor area. A basement with living space, by contrast, does count.
• Attic. Area in an attic counts to the extent that there is sufficient ceiling height — the part of the area under sloping walls where the height is too low is often not counted.
• Usable attic and dormers can shift the area considerably, and here it is worth calculating precisely rather than rounding.
• Carports, garages and outbuildings are treated separately and can be wholly or partly kept outside the floor area, within set limits.

The rules for what counts are set in the building code and can be supplemented or tightened in the local plan. They change over time and are interpreted case by case by the municipality, so they should always be verified against the current edition of the code and with the municipality in question. The difference between the two concepts is important enough to deserve its own walkthrough — see what counts towards the plot ratio: gross floor area vs. footprint.

Step 3: calculate the building rights

With the correct site area and a correct understanding of the floor area, you can turn the formula around and calculate the other way: instead of measuring an existing floor area, you calculate the maximum usable building right.

Maximum floor area = site area × (maximum plot ratio ÷ 100)

A site of 1,200 m² with a maximum plot ratio of 40 therefore has a theoretical building right of 480 m² of floor area. But “theoretical” is the operative word. The usable building right is typically lower than the mathematical maximum, because other provisions limit where and how high the volume can actually be placed:

• Building zones delimit where on the site construction is permitted.
• Height-limit planes, maximum building height and number of storeys cap how much area you can stack within the footprint.
• Setback requirements to boundaries and building lines reduce the area that can realistically be built on.
• Parking and open-space requirements must fit on the site and eat into the available room.

This is where the mathematically possible meets the physically possible. Height is often the hardest constraint — if you cannot go up, you cannot realise the area, regardless of what the plot ratio permits. So always review the height-limit plane and building height to find how high you may build before you treat the building right as realisable.

Step 4: distinguish between maximum and usable building rights

On a developed site there are two figures that matter: the maximum building right and what has already been used. The difference is the residual capacity — the unused building right, which is often the real value in an existing property.

Calculate it by subtracting the existing floor area — which you find in BBR (the Buildings & Dwellings Register) — from the maximum building right:

Residual capacity = maximum floor area − existing floor area

Here you must be careful with BBR. Registered areas are not always up to date, and BBR’s area concepts do not necessarily follow the building code’s floor-area definition one-to-one. So check the BBR figures against floor plans or a survey before you base a purchase decision on residual capacity. A site can look fully utilised in BBR and still hold a significant reserve — or the reverse. This is how you find unused building rights on a parcel via residual capacity when you want to uncover that value systematically.

Step 5: cross-check with easements and what is not in the plan

Even a correct calculation can be upended by something that appears in neither the cadastre nor the local plan. Registered easements can lay building lines, utility routes or covenants across the site that in practice restrict where and how much you may build — independently of the plot ratio.

Always look up the registered burdens in the Land Registry (tingbogen) and read them together with the planning framework. An easement covering a sight line or a protected utility route can significantly reduce the area that can actually be built on, and it only surfaces if you actively bring it up. This is the same discipline that belongs to a thorough due diligence checklist for a building site.

From parcel to building-rights figure in minutes

The manual exercise — looking up the cadastral area, finding the applicable local plan and municipal plan framework, deciding what may be counted into the floor area, subtracting the existing area from BBR and cross-checking with registered easements — typically takes hours per site, and every step holds a source of error. This is precisely the consolidation that Arcili automates: Projektvurdering (project valuation) pulls the public registers — the cadastre, planning data, BBR and the Land Registry — from the same place and calculates both the maximum and the utilised building right, so you can read the real volume of a parcel in minutes instead of an afternoon.

This does not remove the work of the architect or engineer — it saves you the 80-100 hours in the early phase where you have to decide whether the site is even worth bidding on. Want to see how building rights and plot ratio are calculated on a specific parcel? Book a walkthrough.