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Existing buildings: what applies after conversion and change of use

Most of the work in a planning office is existing stock. Piloti first classifies the project under your state’s building code and only then cites the requirement that applies to exactly this intervention, not the one for the new build next door.

In short

For a conversion, extension, change of use or major renovation, Piloti first classifies the intervention with a citation under your state’s building code, checks the building class again and then names the OIB requirements that apply to this project. If a requirement cannot be met in the existing building, Piloti builds the deviation case with protection goal and evidence.

Try it with a real questionChecked: September 2026

What Piloti delivers

The regime with a citation
At the top: what kind of project the intervention counts as, and under which provision.
A case split
If the answer forks at the regime, the cases stand side by side in a table, with your case marked once it is settled. Existing and altered states can be compared as tabs.
A check table
Several requirements, each with its own verdict, its own source and the regime it belongs to.
A note when it changes the next step
A deadline or a reservation stands highlighted above the answer, one at most, so it does not get lost among the others.

Why existing buildings are harder than new ones

Conversion, extension, change of use, major renovation, repair: the words sound related and are not interchangeable in law. Which provisions an intervention triggers depends on how the state’s building code classifies it, and each state does so with its own terms and thresholds.

The OIB guidelines are largely written from new build outwards, and the edition binding today is often not the one that applied at the original permit. A new-build clause applied to an attic conversion reads correctly and is still wrong. That is more dangerous than an obviously wrong number, because nobody stops to question it.

And a conversion can shift the building class. A converted attic with flats becomes an above-ground storey, and the escape level rises. If the class changes, almost every later number changes, from fire resistance to escape routes.

How Piloti works through the question

  1. 01

    Classify the project

    Piloti fetches the provision that distinguishes conversion, extension and change of use in your state and places the project there. If the project type is already recorded, it is read, not overwritten. If it is unclear, a question comes, rather than carrying on as new build and mentioning it at the end.

  2. 02

    Settle what still binds

    Existing rights, the binding OIB edition, whether a major renovation reopens the building envelope: each of these is a clause that can be fetched, not a feeling.

  3. 03

    Check the building class again

    If the intervention shifts the number of storeys, the escape level or the use, Piloti settles the class again before naming a requirement.

  4. 04

    Assign requirements to the regime

    Every requirement cited belongs to the regime that applies to this project, not to the new build next to it.

  5. 05

    Build a deviation as a case

    If a requirement cannot be met in the existing building, Piloti builds the case: which requirement is not met, with its citation; which protection goal it serves; how the same goal is reached in this building; what has to be demonstrated for it.

What Piloti needs from you

  • State and project, in your own words: “attic conversion with two flats”, “the office becomes a medical practice”.
  • The permitted existing state, as far as available: building permit, as-built drawings, earlier conditions.
  • The use before and after.
  • The interventions in structure, envelope and circulation, as far as they are settled.

Good to know

What an existing building lawfully carries, Piloti reads from its permits; if they are not in the project, it says so instead of assuming a permitted state. Piloti builds a deviation case as a sourced argument, and the authority decides whether the same level of protection is reached. Responsibility for the design stays with your office.

Questions

Do new-build requirements apply to a conversion?

Not automatically. Which requirements a conversion triggers depends on how your state’s building code classifies the project and which parts of the building are affected. Piloti settles the regime first and only then cites a requirement.

What is a change of use in building law?

A change in the purpose of a building or part of a building, for example from office to flat. Whether it needs a permit or a notice and which requirements it triggers is set by the state’s building code. It often also shifts the building class or the requirements for fire safety and daylight.

Can an attic conversion change the building class?

Yes. A converted attic with flats counts as an above-ground storey if it lies mostly above ground, and it raises the escape level. Both can push the building into a higher class, with consequences for almost every fire safety requirement.

Does Piloti help with a deviation from the OIB guidelines?

It builds the case: which requirement is not met, which protection goal it serves, how the same goal is reached in this building and what has to be demonstrated. The authority decides on it.

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