The degree of land use
The Luxembourg terminology is precise and set by regulation. Documents refer to the ratios by their French abbreviations.
| Ratio | What it relates |
|---|---|
| Land use ratio (CUS) | total built area to plot area |
| Site coverage ratio (COS) | ground footprint of buildings to plot area |
| Soil sealing ratio (CSS) | sealed ground surface to plot area |
| Housing density (DL) | number of dwelling units to plot area |
These four parameters apply simultaneously. A project may meet the land use ratio and still be blocked by housing density, or by sealing.
The most restrictive determines the project, and it is not always the one looked at first.
The distinction that distorts most calculations
This is the main trap, and it does not show on a quick reading. The parameters combine rather than apply in isolation.
The ratios do not all relate to the same plot area.
Some are calculated on gross developable land, which includes in particular the areas intended for servicing.
Others are calculated on net developable land, from which those are deducted.
The gap between the two can be substantial, the share given over to servicing representing a significant fraction of land intended for urbanisation.
Consequence: applying a ratio to the wrong area produces an error of tens of per cent in the estimated development potential, always in the optimistic direction.
The rule is therefore to check, in the written part of the document consulted, which area each ratio relates to, before any calculation.
The reference area
The central quantity deserves particular attention, as its definition is not intuitive. It determines the developable area.
Gross built area adds the areas of all levels, measured externally, building and outbuildings included.
Certain areas are excluded, notably areas incapable of conversion in the basement or under the roof.
Unenclosed areas are also excluded, such as balconies, loggias and open car ports.
These exclusions are a design lever, since they allow certain spaces to be built without consuming development potential.
But they vary with the document consulted, which requires reading the applicable definitions rather than reasoning from habit.
The siting parameters
They bound volume differently, and are often more decisive than the ratios. They concern heights and setbacks.
Setbacks from plot boundaries and from the road, which define the available footprint.
Heights, expressed against reference points specified in the document, together with the number of levels permitted.
Alignments and building bands, which impose a position and not merely a distance.
Form and roof requirements, particularly in areas subject to an integration requirement.
On a narrow or constrained plot, setbacks determine the project, well before the ratios.
A modulation mechanism worth knowing
One scheme can alter the degree of land use. Its reach is checked in the written part.
The law provides a mechanism increasing the degree of land use in return for commitments on affordable housing, applying to certain urbanisation schemes.
It concerns mainly operations of a certain size rather than individual projects.
Its regime has been amended and is the subject of further proposals, which requires checking the law in force at the time of the project.
What this means for a professional
Four rules.
Check the reference area of each ratio before any calculation, the error being frequent and always favourable in appearance.
Calculate all four ratios, and take the most restrictive.
Check setbacks before ratios on narrow plots, where they decide the project.
Read the definitions in the document consulted, these being capable of differing between documents.
This article reflects the regulatory framework at the date of checking and serves professional orientation. It does not replace consultation of the municipality or legal advice.