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Determining the building class: GK 1 to 5 under OIB

Piloti places your building in GK 1 to 5, with the wording of the OIB definitions as evidence and the edition that applies in your state. If a deciding fact is missing, it asks exactly one question.

Terms from: OIB-Richtlinien, Begriffsbestimmungen, edition May 2023.

In short

Piloti determines the OIB building class (GK 1 to 5) from the escape level, the number of above-ground storeys, the floor area and the use, and for GK 1 and GK 2c also from access on at least three sides, and delivers the class as a copyable value with a checked citation and the edition that applies in the state.

Try it with a real questionChecked: September 2026

What Piloti delivers

The class first
The result comes first as a copyable value, such as “GK 3”, followed by the short reasoning and the assumptions it rests on.
A case split where it tips
If a project sits on a boundary, the cases stand side by side, such as with and without a converted attic.
A section sketch
Where height and escape level are the subject, Piloti draws a schematic section with storeys, ground and escape level.

Why the classification is tricky

The building class follows from four facts, and each has a typical mistake attached. The escape level (Fluchtniveau) is not the building height but the difference between the floor of the highest above-ground storey and the ground after completion, averaged. An unconverted attic does not count as an above-ground storey. GK 1 to 3 use gross floor area, GK 4a the usable area of each individual flat or business unit.

Then there is the edition. The OIB guidelines apply only once a state declares them binding, and the states do so at different times. According to the OIB overview (as of September 2025), the 2023 edition of guidelines 1 to 5 applies in Vienna since 23 February 2024, in Lower Austria since 18 March 2025 and in Upper Austria since 1 October 2025, among others. Where a state has not declared it, the 2019 edition generally still applies. The state’s building-technology rules remain decisive, because they can set exceptions and transition rules.

How Piloti works through the question

  1. 01

    Take the confirmed class

    If the project holds a confirmed building class, Piloti works with it. If your question contradicts it, Piloti says so and stays with the confirmed one until someone changes it.

  2. 02

    Ask for the one missing fact

    If the class is missing and changes the answer, Piloti asks exactly what decides it: escape level, above-ground storeys or use. One question, not a form. If a clearly stated assumption allows a useful answer, Piloti gives it and marks the assumption.

  3. 03

    Pin down edition and state

    Piloti names the edition that applies in the project’s state wherever it moves the result, and cites the passage.

  4. 04

    Classify with a citation

    The classification rests on the wording of the definitions. Every citation is checked against the source text before it is shown.

  5. 05

    Look at the drawing when it decides

    If a section or elevation is in the project, Piloti looks at it as an image and says which drawing on the sheet it read.

What Piloti needs from you

  • The state, or a project that records it.
  • The escape level, or the levels it follows from: finished floor of the highest above-ground storey and the adjoining ground after completion.
  • The number of above-ground storeys, and whether the attic is converted.
  • The use and the number of flats or business units, with their areas.
  • For detached buildings: whether they are accessible from outside for firefighting on at least three sides.

Good to know

Piloti delivers the classification with its citation so your office can check it; responsibility for the design stays with you. A level the drawing does not show clearly is asked for rather than estimated. A borderline project goes into the pre-application meeting with this reasoning in hand.

Questions

How do I determine the building class under OIB?

You need the escape level, the number of above-ground storeys, the floor area and the use, and for GK 1 and GK 2c also access on three sides. According to the OIB definitions, GK 1 to 3 end at three above-ground storeys and an escape level of 7.00 m, GK 4 at four storeys and 11 m, GK 5 at an escape level of 22 m. Which edition applies is set by your state.

Is the building class the same as the Bauklasse?

No. The building class comes from the OIB guidelines and mainly drives the fire safety requirements. The Bauklasse is a term of the Vienna building code through which the zoning plan sets the permitted building height. A Viennese building has both, and they follow from different rules.

How is a terraced house classified?

For terraced houses, the OIB definitions say each flat or business unit is to be considered separately with regard to the escape level. GK 2b covers terraced 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 of the above-ground storeys each.

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