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What is a building class?

Almost every fire safety question starts with the same counter-question: which building class? Here are the five classes under the OIB guidelines, what determines them, and where they get confused with the Bauklasse. Piloti classifies your project, citing the definitions.

Definitions from the OIB-Richtlinien, Begriffsbestimmungen, May 2023 edition, in our own translation.

In short

The building class (Gebäudeklasse, GK 1 to 5) classifies a building under the OIB guidelines by escape level, number of above-ground storeys, floor area and number of dwellings or business units; above all, the fire safety requirements of OIB guideline 2 depend on it. Piloti classifies it from these values, with the citation, and keeps it apart from the Bauklasse of a building code.

Try it with a real questionChecked: September 2026

Building class 4 (GK4)

“a) Buildings with no more than four above-ground storeys and an escape level of no more than 11 m, consisting of several dwellings or several business units, each with no more than 400 m² usable floor area of the individual dwellings or business units in the above-ground storeys, b) buildings with no more than four above-ground storeys and an escape level of no more than 11 m, consisting of one dwelling or one business unit without a limit on the gross floor area of the above-ground storeys.”
Own translation of “OIB-Richtlinien, Begriffsbestimmungen, Ausgabe Mai 2023”

How Piloti classifies a building

  • From escape level, above-ground storeys, floor area and use, citing the definitions and the project’s state.
  • If the project already holds a confirmed building class, Piloti takes it. If a deciding value is missing, it asks for exactly that or states the assumption it works with.
  • It says which OIB edition applies in the state when that changes the class, and answers follow-ups such as “and in GK 4?” from what it has already read.

The five building classes in brief

Paraphrased from the definitions. Where in doubt, the wording decides, not this summary.

GK 1
Detached, accessible for firefighting on at least three sides from its own land or from traffic areas. No more than three above-ground storeys, escape level no more than 7.00 m, no more than 400 m² gross floor area of the above-ground storeys in total, no more than two dwellings or one business unit.
GK 2
No more than three above-ground storeys and escape level no more than 7.00 m, and in addition either no more than 400 m² gross floor area in total, or a row house with units of no more than 400 m² each, or a detached, purely residential building accessible from three sides with no more than 800 m² in total.
GK 3
No more than three above-ground storeys and escape level no more than 7.00 m, where the building does not fall into GK 1 or GK 2.
GK 4
No more than four above-ground storeys and escape level no more than 11 m, with several dwellings or business units of no more than 400 m² usable floor area each, or with a single dwelling or business unit without an area limit.
GK 5
Escape level no more than 22 m, where the building does not fall into GK 1 to 4. A taller building falls into none of the five classes; the OIB guidelines set their own, stricter requirements for it.

Building class is not Bauklasse

The Bauklasse is a term of plot development law, for example in Vienna’s building code: it is set in the zoning and development plan and governs how high one may build on a plot. The building class comes from the OIB guidelines and describes the finished building, so that the technical requirements, fire safety above all, fit it.

The two values sit side by side and are not derived from each other. Anyone who says “class 4” in a meeting should say which one they mean.

What depends on the building class

OIB guideline 2 grades its requirements by building class: fire resistance of load-bearing elements, fire compartments, escape routes, facades. A wrong classification therefore runs through the whole fire safety design.

Which edition of the OIB guidelines applies is decided by the state. According to the OIB overview, the 2023 guidelines have not been declared binding in every state; where they have not, the 2019 edition generally still applies. When in doubt, check your state’s building-technology rules, because states can set exceptions and transition rules.

Good to know

The classification remains your decision as the responsible planner; Piloti lays out the definition, citation and assumptions for it. It takes the escape level from your documents, and if a deciding value is missing, it asks for it.

Questions

What is building class 4?

Building class 4 covers buildings with no more than four above-ground storeys and an escape level of no more than 11 m. They consist either of several dwellings or business units of no more than 400 m² usable floor area each, or of a single dwelling or business unit without an area limit. That is how the OIB definitions, May 2023 edition, put it.

How is the building class determined in Austria?

You establish the escape level, the number of above-ground storeys, the gross or usable floor area and the number of dwellings or business units, then check the classes in order, from GK 1 upwards. The OIB edition that applies in the project’s state is the one that counts.

Is building class the same as Bauklasse?

No. The Bauklasse is set in the development plan and, under the building code, for example in Vienna, governs the permitted building height. The building class comes from the OIB guidelines and mainly determines fire safety requirements. A building has both values, independently of each other.

Which building class is a row house?

The GK 2 definition expressly names row houses with no more than three above-ground storeys, an escape level of no more than 7.00 m and units of no more than 400 m² gross floor area each. The escape level is considered separately for each unit.

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