You have found the ideal apartment. The advertised floor area meets your needs, the rooms are well fitted out and everything appears to be in order. However, what is visible does not always match the situation recorded in the official co-ownership documents.
Insufficient document checks can turn what appeared to be a 162 m² duplex into an 88 m² apartment with an attic. That difference can have significant consequences when buying, financing or reselling the property.
The vertical cadastre can be used to check the official designation, usable floor area and ownership share of each unit in a co-owned building. When a space recorded as an attic is used as living accommodation, regularising it may depend both on the unanimous agreement of the co-owners and on the planning rules applicable in the municipality.
Key Takeaway
In a co-owned building, a fitted, heated room used as living space is not automatically recognised as residential accommodation. The official designation of the unit may be just as important as its floor area.
What is the vertical cadastre?
The vertical cadastre is specific to buildings subject to co-ownership rules. It divides the building into privately owned units and identifies the cadastral status of each one.
For each unit, it generally records several details:
- A reference number.
- The official designation of the unit, such as apartment, cellar, attic, garage or parking space.
- The usable floor area assigned to the unit.
- Its share of the common parts, expressed in thousandths.
- Its location within the building.
These thousandths are used, among other things, to determine each co-owner’s share, allocate many common charges and calculate voting rights at general meetings.
Does the vertical cadastre apply to single-family houses?
No. The vertical cadastre applies to buildings divided into co-owned units. It is therefore particularly relevant when buying an apartment, duplex, triplex, cellar, garage or parking space in a residence.
In a single-family house, a converted basement, unauthorised annex or extension can also raise important issues, but these relate primarily to planning compliance and municipal permits. That is a separate subject.
Why is the unit’s official designation so important?
During a viewing, a buyer naturally assesses the spaces that are visible. A room with windows, heating, flooring and internal access can clearly appear to form part of the accommodation.
However, everyday use does not automatically change a space’s official classification. A converted upper floor may still be recorded as an attic in the vertical cadastre and the deed of division.
Common official unit designations
- Apartment
- Retail or office unit
- Cellar
- Attic
- Garage
- Indoor or outdoor parking space
These designations are not interchangeable. A converted attic should not be treated as recognised living space without verification.
Key Takeaway
The floor area shown in a property listing should be compared with the official designation of the units. The physical existence of a room is not enough to establish that it can be valued as living space.
A real example: the 162 m² duplex that became an apartment with an attic
A couple views an attractive duplex advertised as offering 162 m² of living space. The property meets their expectations and they proceed with the purchase.
Several years later, the owners decide to sell and request a valuation. During the document review, an important discrepancy appears: the upper level is recorded as an attic in the vertical cadastre.
The questions that then arise
- Was the conversion authorised by the municipality?
- Did the co-ownership approve the change of use?
- Were the deed of division and co-ownership rules amended?
- Can the upper floor genuinely be presented as residential accommodation?
- Can the situation still be regularised?
What was purchased and maintained as a 162 m² duplex may then have to be analysed as an apartment of approximately 88 m² with a converted attic. The potential effect on valuation and resale can be considerable.
Why is the issue often discovered during resale?
At the time of purchase, attention often focuses on the condition of the accommodation, the finishes, the neighbourhood, common charges and borrowing capacity. Technical and legal documents may be reviewed quickly or without a detailed comparison.
During resale, a new agent, buyer, bank, notary or other professional may compare:
- The visible layout of the property.
- The advertised floor area.
- The vertical cadastre.
- The deed of division.
- The approved plans.
- Information available from the municipality or co-ownership.
The original buyer may then discover an issue that already existed at the time of purchase but was not identified or sufficiently explained.
Can a converted attic be regularised as living space?
Regularisation may sometimes be possible, but it is neither automatic nor guaranteed. It may require several independent procedures and the involvement of different professionals.
In this example, two separate aspects must be considered:
| Aspect | Main question |
|---|---|
| Co-ownership | Do the co-owners agree to change the designation of the privately owned unit? |
| Planning | Can the municipality authorise the use of the space as residential accommodation? |
Approval under one aspect does not guarantee approval under the other.
Changing the unit’s designation: unanimous approval
When an attic is to become an official part of the accommodation, the process may require the designation of the privately owned unit to be amended in the deed of division or the co-ownership rules.
A change in the designation of privately owned parts is treated as a decision requiring unanimity. All ownership shares, representing 1,000/1,000, must therefore vote in favour.
A single opposing co-owner may block the process, even where that person’s share is limited to a garage or parking space.
Do not confuse building works with a change of designation
- Improvement, alteration or addition works may be subject to a different qualified majority, depending on their nature.
- Changing the official designation of a privately owned part requires unanimity.
- A change to the allocation of charges or ownership shares may also require unanimity.
In a specific case, the co-ownership rules, deed of division, exact nature of the proposed change and any works should be reviewed with the property manager, notary and other relevant professionals.
Key Takeaway
If regularisation requires an attic to be officially redesignated as living space, unanimous approval from the co-owners is required. A single opposing vote may prevent the change.
Even with unanimous approval, the municipality may refuse
Co-ownership approval does not replace planning permission. The municipality must still determine whether using the space as residential accommodation complies with the applicable rules.
The assessment may cover:
- The General Development Plan, or PAG.
- The Detailed Development Plan, or PAP.
- The maximum authorised number of habitable levels.
- Permitted heights and volumes.
- Requirements applicable to habitable rooms.
- Natural light and ventilation.
- Municipal rules in force when the application is submitted.
Example: a fourth level that cannot be authorised
Suppose the applicable PAP authorises a maximum of three habitable levels. The duplex attic is located on the fourth level, within the roof space, and using it as accommodation would create an additional habitable level.
In that case, even if every co-owner agrees to the change of designation, the municipality may be unable to regularise the space as residential accommodation under the PAP.
The fact that the conversion has physically existed for several years does not automatically create a right to regularisation.
Key Takeaway
The fact that a room has existed and been used for 10, 15 or 20 years does not mean that it is compliant or capable of being regularised. Unanimous approval from the co-ownership does not guarantee municipal permission.
What may be the consequences for the owner?
Every situation must be assessed individually. However, a discrepancy between the property’s layout and the official documents may create several difficulties.
- The advertised living area may be called into question.
- The property may require a different valuation.
- Buyers or their bank may request clarification.
- Steps may be required with the property manager, co-ownership, notary, surveyor or municipality.
- Additional costs and delays may arise before the property can be marketed.
- It may be impossible to present the entire space as residential accommodation.
Identifying the issue before purchase allows the buyer to make an informed decision, ask the right questions and, where necessary, adjust the price or conditions of the acquisition.
Which documents should you request before buying?
The vertical cadastre should not be read in isolation. The aim is to check consistency between several documents and the visible situation.
Documents worth comparing
- The vertical cadastre.
- The deed of division.
- The co-ownership rules.
- Plans attached to the co-ownership documents.
- Plans approved by the municipality.
- Permits relating to any alterations.
- Minutes of general meetings that may mention the works or changes.
If a discrepancy appears, it is better to clarify it before signing the preliminary sale agreement than to rely on a later regularisation.
The 7 essential checks before signing
| Check | What it helps establish |
|---|---|
| 1. Unit designation | Whether the spaces are officially recorded as an apartment, cellar, attic or another type of unit. |
| 2. Floor areas | Whether the documented areas are consistent with those used in marketing. |
| 3. Thousandths | The ownership share attached to each unit and its role in charges and voting. |
| 4. Plans | Whether the visible layout corresponds to the available plans. |
| 5. Permits | Whether alterations were approved by the authorities and, where required, the co-ownership. |
| 6. Planning feasibility | Whether the PAG, PAP and municipal rules would allow regularisation. |
| 7. Co-ownership majority | Whether the proposed change requires unanimity or another qualified majority. |
The BLImmo Approach
When valuing or marketing an apartment, the analysis should not be limited to a viewing and a price-per-square-metre calculation.
The available documents may reveal information that is just as important as the property’s visible condition. A well-converted space does not necessarily have the same value or status as an area officially recognised as residential accommodation.
Check before you value
At BLImmo, we pay particular attention to consistency between the situation observed, the vertical cadastre, the co-ownership documents and the information used to present a property.
When a discrepancy appears, the aim is not to jump to conclusions, but to identify the checks required before setting a value or marketing the property.
FAQ: Vertical Cadastre in Luxembourg
What is the vertical cadastre?
The vertical cadastre identifies the privately owned units in a co-owned building, their designation, usable floor area, location and share of the common parts.
Does the vertical cadastre apply to a single-family house?
No. It applies to buildings subject to co-ownership rules. Issues involving an annex, extension or converted basement in a single-family house primarily concern planning compliance.
Can a converted attic be counted as living space?
Not automatically. Its designation in the co-ownership documents, the plans and the municipal permits must be checked. Physical conversion alone does not establish that the space is officially recognised as residential accommodation.
What majority is required to officially convert an attic into living space?
Where the process involves changing the designation of a privately owned part in the co-ownership rules or deed of division, unanimous approval from the co-owners is required. The specific file and co-ownership documents should nevertheless be reviewed.
Does unanimous co-ownership approval guarantee regularisation?
No. The municipality must still be able to authorise use of the space as residential accommodation under the applicable PAG, PAP and municipal rules.
Is a room that has been used for many years automatically capable of being regularised?
No. The age of a conversion guarantees neither its compliance nor the possibility of regularising it. The rules applicable at the time of the application must be checked.
When should the vertical cadastre be checked?
As early as possible, ideally before signing the preliminary sale agreement. Any discrepancy between the documents and visible layout should be clarified before the buyer becomes definitively committed.
Our Local Expertise
BLImmo supports owners and buyers in western Luxembourg, including Steinfort, Hobscheid, Koerich, Mamer, Kehlen, Capellen, Garnich, Eischen, Windhof and the surrounding municipalities.
Our approach combines a structured property analysis, careful review of the available documents and knowledge of the local market so that every property can be presented consistently and transparently.
Sources and Further Information
Would you like a valuation of an apartment or duplex?
A coherent valuation is not based solely on the advertised floor area. The designation of the units, co-ownership documents and compliance of the layout also deserve careful attention.
BLImmo can assist with the analysis of your property, its documentation and its positioning in the local market.