Check your case
At most three questions, and you will know whether you need a building permit or a main design is enough.
Your case
No building permit, built to a main design
A one floor outbuilding of up to 50 m² footprint, on the plot of an existing house and serving it, is built without a building permit but to a main design. It must comply with the spatial plan, including site coverage and distances from the boundaries.
- Main design for the outbuilding
- Notice of start of works before building
- Compliance with the spatial plan
Your case
A building permit is needed
An outbuilding larger than 50 m² of footprint or with more than one floor is outside the exemption in the rulebook, so it needs a building permit, just like a house.
- Design for the building permit
- Check of site coverage and floor area ratio
- Land register extract as proof of legal interest
Your case
On the same permit as the house
If you are only now building the house, the outbuilding can be part of the same design and the same permit. In the design it is shown as a separate building, clearly apart from the house, with its own area.
- One design for the house and the outbuilding
- One permit procedure
- One check of site coverage
Your case
A building permit, if the plan allows it
The exemption for outbuildings up to 50 m² applies only on the plot of an existing building or one with a building approval. On an empty plot an outbuilding needs a building permit, and the spatial plan has to allow it without a house.
- Check of the spatial plan for the plot
- Design for the building permit
Your case
A building permit for the alteration
Extending a house or adding a floor counts as reconstruction: it changes the outer dimensions and the house's compliance with the location conditions. It needs a building permit, and the spatial plan must allow the larger area, height or number of floors.
- Drawings of the existing house or a survey of it
- Check of coverage, floor area and floors under the plan
- Reconstruction design for the building permit
Your case
Check the house documents first
Only an existing building can be extended, that is one built under a permit or treated as such, for example one built before 15 February 1968. So first check which documents the house has.
- Occupancy or building permit
- Legalisation decision
- The 1968 aerial image on the Geoportal
Your case
The existing house has to be sorted out first
Reconstruction only applies to an existing building, one built under a permit or treated as such. For a house without these documents an extension cannot be approved until its status is resolved.
- Check whether some act for the house exists after all
- Options for the old house, including removal and a new build
Your case
Often without a permit, to a main design
Work on an existing house that improves how it meets the basic requirements, and a new layout that does not affect the structure or fire safety, is done without a building permit to a main design, as long as the house's compliance with the location conditions does not change. Some smaller works need no main design at all.
- Description of the work and photos of the house
- Main design, where the work requires it
- Notice of start of works, where a main design is used
1. Only a legal house can be extended
The law treats extensions and added floors as reconstruction, and reconstruction applies to an existing building. Existing means built under a building permit or another appropriate act, or treated as such, for example a building built before 15 February 1968.
So the first step is always the same: which documents does the house have. Without them an extension cannot be approved until the status of the house is resolved.
2. Extensions and added floors need a building permit
Extending, adding a floor, removing an outer part of the house and changing its use all change its compliance with the location conditions. They need a building permit, as a new build does.
The spatial plan must also allow the larger house. If the plot is already close to its maximum coverage or number of floors, there is no extension, whatever the documents.
In a reconstruction at least the foundations and part of the lowest floor must stay. If more is removed, it is no longer a reconstruction but a new build.
3. Work that does not enlarge the house
Work on an existing house that improves how it meets the basic requirements is done without a building permit, to a main design, as long as it does not change compliance with the location conditions. The same goes for a new layout that changes rooms, partitions or services without affecting the structure or fire safety.
Before such work starts, the start of works is notified. Some smaller works need no main design at all.
4. An outbuilding up to 50 m² is built to a main design
On the plot of an existing building, or one with a building approval, and serving it, a one floor outbuilding of up to 50 m² footprint can be built without a building permit, to a main design. The rulebook treats a few other ancillary structures the same way.
All of them must comply with the spatial plan. An outbuilding counts towards site coverage, and the plan also sets how close to the boundary it may stand.
| Structure | Condition |
|---|---|
| Outbuilding | One floor, up to 50 m² of footprint |
| Swimming pool | Sunk into the ground, up to 100 m² |
| Fuel tank | Up to 10 m³ |
| Solar collectors and panels | No feeding into the grid |
All on the plot of an existing building and serving it, in line with the spatial plan.
5. Larger outbuildings and outbuildings with a new house
An outbuilding larger than 50 m² of footprint or with more floors needs a building permit. So does an outbuilding on a plot without a house, if the spatial plan allows it at all.
If you are only now building the house, the outbuilding can go on the same permit. In the design it is clearly separate from the house, with its own area.
With an existing house
- Up to 50 m², one floor
- No permit, built to a main design
- Larger or more floors
- Building permit
With a new house
- Up to 50 m², one floor
- On the same permit as the house
- Larger or more floors
- On the same permit as the house
6. What to send us for an offer
For an offer we need the parcel number, the cadastral municipality and a short description of your plans. For an extension we also need drawings of the existing house. If you have none, they are made from a survey of the existing state.
We first check what the spatial plan allows on your plot, and only then start drawing.
For an offer, send us
- Parcel number and cadastral municipality
- A short description and rough size
- Documents and drawings of the existing house, if you have them
Common questions about extensions and outbuildings
Does an outbuilding count towards site coverage?
Yes. The site coverage ratio compares the land covered by buildings with the plot area, and an outbuilding is one of those buildings.
Source · Rulebook on spatial plans, art. 4 ↗
May an outbuilding stand on the boundary?
The spatial plan for your zone decides. Some plans allow outbuildings closer to the boundary than the house, others do not. We check it for your plot.
Is a new rulebook coming?
The new Building Act calls for a new rulebook on what may be built without a permit, and its draft was in public consultation until 3 September 2026. Until it takes effect, the current rulebook applies. We will update this guide when the rules change.