Height-Limit Plane: How High You Can Actually Build

You can have all the plot ratio you want and still be unable to build it if the height-limit plane slices the corners off the volume. In most low-rise-dense and multi-storey projects it isn’t the plot ratio that sets the ceiling — it’s the height. On paper a plot might support 60% built coverage, but if the sloping height-limit plane forces the top floor in toward the centre of the plot and cuts away the corners of the roof, the realisable area disappears fast.

It is one of the constraints that surprises people most often at the sketch stage, because it isn’t a single number but a geometry. The plot ratio is a simple fraction you can work out on a piece of paper. The height-limit plane and the building height, by contrast, are an interplay between a fixed maximum height, a permitted number of storeys, and a sloping plane that rises from the boundary. Understand those three quantities and the order in which they must be read, and you can determine the real number of storeys a plot allows before an architect has drawn the first line.

The three quantities that together set the ceiling

How high you may build is rarely decided by a single provision. Three constraints act at the same time, and the one that bites first is the one that governs:

• Maximum building height. An absolute ceiling in metres, measured from a fixed datum plane to a defined point on the building — typically the top of the roof or a specific height of the façade. Usually set in the local plan.
• Maximum number of storeys. The number of permitted storeys, e.g. “2 storeys with a usable roof storey”. This figure can limit the volume even where the height in metres would allow more — and vice versa.
• The sloping height-limit plane. A sloping plane that rises inward from the neighbouring boundary and lays a declining maximum height across the plot. It is this geometry that “slices the corners off” the uppermost volume close to the boundary.

The smallest of the three governs. A local plan may permit 8.5 metres and two storeys, but if the building stands close to the boundary, the height-limit plane can in practice force the upper part further down. So always read all three together — and find the specific values in the local plan, where building height, number of storeys and use are set.

How the sloping height-limit plane works

The sloping height-limit plane is the principle that a building must keep its distance from the neighbour vertically, not only in plan. The idea is to secure light and air toward the neighbouring boundary: the closer you build to the boundary, the lower you may build.

In practice it works like this: from a point in or above the neighbouring boundary, a sloping plane rises in over your plot. The building must not break through this plane. The plane is defined by two things — a starting point (a height at or just above the boundary line) and a slope (how steeply the plane rises in over the plot, typically expressed as a ratio between height and horizontal distance).

The consequence is geometric: the further the building is pulled back from the boundary, the higher it may be, because the sloping plane has risen accordingly. That is why a set-back top floor or a hipped/sloping roof is often the key to getting volume out of a plot that is squeezed by the height-limit plane — you simply follow the plane’s slope with the building’s silhouette.

Rule of thumb: every metre you move the building away from the boundary typically buys you part of a metre of extra permitted height along the height-limit plane. If you want to go up close to the boundary, you have to set the floor back instead.

The specific figures for the starting point and slope are set in the building code and can be changed or tightened in a local plan. The rates have also changed over time and are interpreted case by case by the municipality, so they should always be verified in the current edition of the code and in the local plan for the individual plot — never calculate from a figure you remember from an earlier project.

How to calculate the maximum building height at a point

The sloping-height-limit-plane calculation is about finding how high the plane sits at the point where your façade or roof edge stands. The method is the same regardless of which rates apply:

1. Establish the datum plane. Heights are measured from a fixed datum plane, not from the arbitrary terrain beneath the building. On sloping plots, fixing the datum plane is decisive and should be clarified early — it shifts every subsequent number.
2. Measure the horizontal distance from the boundary to the point on the building you want to test (typically the nearest, highest corner toward the boundary).
3. Add the height-limit plane’s starting point to the distance multiplied by the slope. The result is the maximum permitted height at exactly that point.
4. Compare with the absolute maximum height. If the sloping plane allows more than the fixed maximum height, it is the maximum height that governs — and vice versa.

The point is that the maximum building height is not one number for the whole plot, but a contour that follows the distance to the boundary. That is why a simple “8.5 metres” reading can be misleading: it applies only where the building stands far enough from the boundary for the sloping plane to allow it. Where you may place the volume in plan is a related discipline — see building zone and building lines: where on the plot you can actually build, when you need to translate the height contour into a physical building zone.

From height in metres to a realisable number of storeys

It is tempting to translate a maximum height directly into storeys, but the relationship is not mechanical. The number of storeys in the local plan is often the binding constraint, and even where it isn’t stated, storey heights set the limit for how many storeys a given height in metres can hold.

When you need to determine the realisable number of storeys, keep these factors in mind:

• Internal storey height. Dwellings require a certain clear ceiling height, and on top of that come the floor slab, the floor build-up and installations. A “raw” storey therefore eats up markedly more of the height in metres than the clear ceiling height alone suggests — commercial use, and retail in particular, typically requires even more.
• Roof form. A usable roof storey beneath a sloping or hipped roof can hold an extra storey without breaching the maximum height, whereas a flat roof uses the height more “outright”.
• Datum plane and terrain. On a sloping plot, a basement can present as a partly exposed plinth storey at the low end — which affects both the perceived height and what counts.

The realisable number of storeys is thus the lowest common denominator of what the height in metres allows, what the number of storeys in the plan allows, and what the sloping height-limit plane allows along the boundary. That is the figure that really decides the volume — and thereby the building rights in practice. Height is inseparably tied to the area calculation; see how you calculate building rights and plot ratio on a cadastral parcel to combine the two into a single volume you can keep working from.

Height toward the boundary and the provisions that aren’t in the local plan

The sloping height-limit plane governs the height toward the neighbouring boundary, but it isn’t the only provision that pushes the volume toward the edge of the plot. Remember to cross-check the height calculation with:

• Setback requirements from the boundary, which can force the building further onto the plot — which, paradoxically, can give more height, because you are further from the height-limit plane’s starting point.
• Registered easements in the tingbogen (the Land Registry), which can lay explicit height restrictions, sightlines or building lines across the plot independently of the local plan.
• Road and sightline conditions, which can limit the height in particular corners of the plot.

Always look up the registered encumbrances in the tingbogen and read them together with the planning basis. A height covenant or a preservation provision can reduce the realisable volume considerably, and it only surfaces if you actively retrieve it.

From height geometry to realisable volume in minutes

The manual exercise — establishing the datum plane, reading the maximum height and number of storeys in the local plan, calculating the sloping height-limit plane along the boundary, translating the height in metres into a realisable number of storeys, and cross-checking the whole thing against registered height restrictions — takes time per plot, and because height is a geometry rather than a single number, it is easy to misjudge in a quick estimate. That assembly is precisely what Arcili automates: Projektvurdering (the project-valuation module) pulls the public registers — Plandata.dk planning data, the matriklen (the cadastre), BBR (the Buildings & Dwellings Register) and the tingbogen — in one place and calculates what the plot can carry if it stood finished today, so you can read the realisable volume before the sketch stage instead of discovering the height-limit plane once the drawing is already done.

It does not remove the architect’s work of shaping the building within the contour — it saves you the 80–100 hours in the early phase spent determining whether the plot carries the number of storeys your calculation assumes in the first place. Want to see how height and volume are calculated on a specific cadastral parcel? Book a walkthrough.