When a municipality allows "a plot ratio of 0.6", it sets the amount of floor area you can build relative to the size of your plot. It is one of the most powerful — and most misunderstood — levers of building rights, because three closely related acronyms coexist in Switzerland.
What a utilisation index measures
A utilisation index is a ratio: the permitted floor area divided by a reference area of the plot. An index of 0.5 on 1,000 m² allows 500 m² of floor area (depending on the definition in the regulations). The higher the index, the higher the permitted density.
Mind the denominator: depending on the regulations, the reference area is not always the total cadastral area. A portion in a non-buildable zone, encumbered or ceded, may be excluded — which changes the calculation.
The index on its own says nothing about the shape of the building: it has to be combined with the building envelope (height, storeys) and the boundary setbacks to obtain a real volume.
IUS, CUS, IBUS: the nuances
| Acronym | Name | Particularity |
|---|---|---|
| IUS | Plot ratio / utilisation index | Common label; its exact definition is set by the municipal regulations |
| CUS | Floor area ratio (FAR) | Vocabulary variant; the chosen label varies by canton and municipality |
| IBUS | Gross floor area ratio | Defined by the intercantonal harmonisation of construction terms (IVHB); authoritative only where the canton or municipality has adopted it |
The IBUS aims to harmonise what counts towards the floor area across cantons — but it only applies where the canton or municipality has written it into its law. Many municipalities still reason in terms of plot ratio / FAR with their own definitions, and some cantons (Geneva) reason mainly in terms of density or gross floor area. Above all, a net index (IUS) and a gross index (IBUS) cannot be compared figure for figure: the gross index encompasses more surfaces (walls, certain circulation areas), so its values are higher at equal density. Always check the label and the definition adopted by YOUR municipality.
Calculating your potential, step by step
Note down the area of your plot
It comes from the official survey (cadastre) and appears in the land register (in m²).
Identify the index applicable to your zone
It is set by the municipal building regulations, zone by zone.
Multiply
Plot area × index = maximum floor area (depending on the definition in the regulations).
Deduct the existing built fabric
Subtract the area already built to estimate the balance still available.
What counts (and does not count) towards the floor area
This is where most errors arise. Depending on the definition adopted, certain elements do or do not count towards the determining floor area: habitable basements, attics, surfaces below a minimum height, balconies, technical rooms, circulation areas.
Beyond the index
A favourable index does not guarantee a large building: the maximum height, the number of storeys and the boundary setbacks can make part of the potential unusable. The index is a ceiling, not a promise. To put the index back into the full set of rules, see the guide knowing a plot's building rights.
Parcellis reads off the zone, the applicable index and the building-envelope constraints for your plot, and clearly indicates what is confirmed and what calls for a check against the regulations.