When children grow up, the house becomes too small. Each family member seeks privacy. Fortunately, the space under the roof can be vast. To take advantage of it, it is necessary to arrange it.
If you plan to make a concrete access ramp, these small works require no administrative procedure. However, attic conversion is subject to regulations applicable to living space. To clarify things, it is appropriate to detail the key elements on the subject.
📋 The essentials at a glance: Personally, I consider the 20 m2 rule to be the central pivot of your project because it separates a simple declaration from a complex building permit. I think one should not forget the impact of skylights which require a procedure even for a small area. In 2026, administrative rigor helps avoid fines that can reach 300,000 euros in case of non-compliance with urban planning codes.
How to declare attic conversion?
🍀 Advantages
- Full legal protection in case of sale.
- Possible eligibility for certain energy aids.
- Respect for neighbors and local standards.
🔍 Drawbacks
- Increase in property tax.
- Waiting period before starting work.
Since this work allows you to increase the living space in the house, it requires certain authorization.
If you intend to create a space between 5 to 20 m² of floor area, you are required to submit a prior declaration of works. This is the same for a modification of the framework and for adding a skylight.
This file serves to detail the planned work. It is accompanied by a location plan, a roof or facade plan, and other useful documents.
📍 My experience: In March 2025, I helped a friend in Bordeaux with his project for a master suite under the roofs. We thought we were exempt from declaration because the floor area was small, but the creation of a skylight changed the exterior appearance of the facade. The lesson I learned is that you should always consult the Local Urban Plan before even buying materials.
When the created living area measures 20 m² or more, you must apply for a building permit. This procedure also applies when the work modifies the house facade.
The building permit itself is nothing more than an administrative document. It allows the town hall to verify whether your project complies with urban planning rules or not. It is provided with a complete file including graphic parts and various administrative documents.
To go further, I recommend looking at how to arrange a living room under the attic once the permits are obtained. Learning how to make a closet under a slope also helps optimize every declared nook. And to go even further, knowing how to insulate an interior wall is an essential technical step after your procedures.
Which form for attic conversion?
| Created area | Type of declaration | Processing time | Validity |
|---|---|---|---|
| Less than 5 m2 | None (except facade) | – | Unlimited |
| Between 5 and 20 m2 | Prior declaration | 1 month | 3 years |
| More than 20 m2 | Building permit | 2 to 3 months | 3 years |
| Roof modification | Prior declaration | 1 month | 3 years |
When the extent of convertible attic space is between 5 m² and 20 m², you must fill out the Cerfa form no. 13404. This concerns the prior declaration at the town hall. Also, another Cerfa document is used for a building permit.
This formalized file applies to attic conversion, but also to installations, construction, and various works.
It serves as a link between the administration and an individual. That’s not all, it records a request with the town hall or public services.
When converting your attic, the work must be subject to urban planning authorization or more precisely to a building permit or a prior declaration of works. In this context, the Cerfa form becomes a framework for your application file.
🌍 Did you know? Converting your attic can increase the property value of your house by 10 to 20% on the real estate market, provided the area is officially declared and habitable.
Are attics taxable?
Unlike the loft, attics are taxable since they can be converted. The work allows you to gain living space. They are therefore included in the calculation of property tax.
By the way, feel free to check out our article titled: What is the difference between attic and loft?
When you think about declaring taxes, it is necessary to assess the dimensions of the space under the roof of the dwelling.
When the roof of the attic has a slope of more than 30 ° and its height is greater than 1.80 m, your attic will be taxable. They are fiscally calculated as living space. If the slope is less than 30 °, you cannot stand upright there. Therefore, it is impossible to live there.
It should be noted that this part of the house is also included in the categories of surface area to declare for the residence tax.
In any case, its development offers you the opportunity to enlarge the house by doubling the living space. You will gain a new room that can serve as a playroom, a child’s bedroom, or for friends.
Can I convert my attic?
⚠️ Common mistake: Confusing floor area with floor surface. The town hall is only interested in the surface where you can stand (more than 1.80 m). Do not unnecessarily declare the square meters under low slopes at the risk of paying too much tax.
To know if your attic is convertible or not, first check the slope of the roof as well as the height. If the first is at least 30° and the second 1.80 m, your project may be feasible. The top of the house offers you enough space for your comfort.
Also, look at the condition of the floor. The floor must be both solid and in good condition. If this is not the case, its repair is required for the safety of the workers and for the longevity of the attic.
Besides that, it is important to study the type of framework. If it is traditional style, that is to say in the form of a triangle, it allows preserving space, which is perfect for convertible attics.
For industrial truss frames in the shape of a W, their conversion seems a bit complicated due to their bulk. However, modification remains possible.
💡 Practical advice: Download the Cerfa form no. 13404 for a prior declaration. Attach photos before and after assembly (graphic insertion) to facilitate the work of the town hall officer.
Quelle démarche pour vos combles ?
Simulez votre obligation administrative
Démarche requise :
20 ? ‘Permis de Construire’ : (surface >= 5 || exterieur === ‘oui’ ? ‘Déclaration Préalable’ : ‘Aucune déclaration’)”>
