One more room, how to do it?

Lay out the attic, install a veranda, transform the garage into a bedroom ... tempting solutions to increase the surface of your house at a lower cost. Provided you respect certain rules.

Want to push the walls to enlarge your house ? The solutions exist to win 10, 20 m2 or more, without embarking on heavy work. In addition, your home will come out grown ... in every sense of the word, since its heritage value will be revalued. It is still necessary to know, before you start, what you really have the right to do, and under what conditions. loft or converted garage, installation of a veranda... the rules are the same.

Direction the town hall to check the feasibility of the enlargement

First indispensable step : to return to town hall to consult the Local Urban Plan (ENJOYED). It is this document which now fixes the local requirements about the height and the appearance of buildings - type of cover, colors and materials of openings, etc.

If your municipality does not have a PLU, as it is still the case in some rural areasyou will need to refer to land use plan (POS). With a difference in size: the POS provides for a land occupation coefficient (COS) which defines the buildable living space maximum depending on the size of your ground. Check that your project is compatible with these requirements.

Declaration of work or building permit for one more room?

Once these first checks have been made, it is necessary to mount the file. Because you will be exempt from any formality if the surface of your project is less than 5 m2. This is little…

Beyond and up 40 m2 of additional floor area (20 m2 in the communes with only one POS), a simple declaration of work filed in the town hall is sufficient (Cerfa form n ° 13404 * 06 to download online). Do not be tempted to simplify your life by skirting this step. This would expose you to a fine or even a deconstruction. Some town halls are very vigilant ... and many denunciations.

For any enlargement of more than 40 m2 (or 20 m2 in case of POS), it becomes necessary to file an application for building permit. Same thing if your work has the effect of bringing the total floor area of ​​your home to more than 150 m2.

The file is then more complex to assemble and the recourse to an expert can be judicious. Once your file is filed, you must receive an answer within two months.

An architect is required under conditions

In the same logic, beyond 40 m2 of expansion or 150 m2 of total area, you must resort to a architect to pilot your project. And if the surface of your housing already exceeds 150 m2, any expansion of more than 5 m2 requires the filing of an application for a building permit and the use of an architect. Know that in any case, his expertise can be valuable. Optimization of the surface, validation of the technical choices, dialogue with the companies, follow-up of the works, its intervention and its professional insurance are guarantees of safety, for fees which represent from 10 to 15% of the cost of the building site.

Budgets and helpers: Things to know before expanding your home

Even if it is not mandatory, consider subscribing to property damage insurance. Against 3.5% of cost of constructionit is essential to protect you from any malfunction and potential litigation.


In enlarging the living space of your home, you also increase his heritage value. Remember to inform your tax Center within three months after the end of the project, so that the IRC re-evaluates your property tax and your housing tax.


Be attentive to VAT rate on the quotes and invoices presented by the professionals. Indeed, thermal insulation of the roof and walls or the laying a roof window benefit from reduced rate to 5.5%. The intermediate rate of 10% applies to renovation : painting, laying wallpapers or sanitary, if they are made in a housing completed for more than two years. Work affecting the structure (masonry, frame, roof, etc.) is taxed at the normal rate of 20%.


Financial aid. The insulation of the roof and the walls are part of the expenses still eligible for the 30% of tax credit for the energetic transition (CITE) in 2018, without means test. Expenditure is capped at 8 000 € for one person alone and 16 000 € for a couple subject to taxation town (+ 400 € per dependent child), all over a period of five consecutive years. You can also claim a Eco-loan at zero rate (eco-PTZ) which makes it possible to borrow up to 30 000 € without interest for energy renovation worksand then repay it over ten or fifteen years, depending on your income. In addition, some of the work involved will also entitle you to energy saving certificates (C2E). Also called "energy bonuses", they represent up to several hundred euros per operation.

What cost to make a room habitable?

This is the average cost to provide to make a habitable room. This range is explained by the differences in implementation. And each site must be assessed on a case by case basis. So, for theattic, drilling the floor in order to pass a staircase or the installation of roof windows are expensive operations. If these openings already exist, the cost can go down to 800 € per sqm.

To enjoy a veranda or a single storey extension, use a wood structure rather than PVC or aluminum can double the bill. And if a soil study is required, its price will be at least 200 €.

For transform the garage in the living room, the transformation will be more economical if the house meets the latest technical regulations (RT 2012), which provides thermal insulation. Otherwise, the score will double, at least.

Video: Anthrax - Room for one more (December 2019).


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