Home expansion: 6 mistakes to avoid


To achieve its goals, the house extension must be perfectly suitable for needs having motivated it and in harmony with the part already built. The first mistake to avoid, before even thinking about the means to implement, therefore consists in neglecting the precise definition of family and environmental aspirations. To synthesize, you have to look for “Why make an enlargement, before to know if it is “feasible and how“.

1 – Badly define your project: source of serious disappointments

Two motivations can be at the origin of an extension:

A simple aesthetic concern, can lead to adapt the house to its environment or to the personal tastes of the occupants. This is often the easiest option to manage, because to improve the style of the building, it is sometimes sufficient to modify the slope of a roof, to add a utility room, to build a courtyard, a veranda, a tower or garage …).

Adapting the living environment to a new imperative, such as the arrival of a new occupant or a change of activity, avoiding the inconvenience of a move, remain the most frequent motivation for enlarging houses. In this case, the integration of the new building must not only meet the aesthetic requirements, but comply with the standards of comfort and of use envisaged. The administrative procedures can also be different.

In quality of master of work, you alone, are responsible before the law and can define the principle orientation for the extension to be carried out. The architect, the project manager and the contractor, however competent and talented they may be, can only advise you and help you on the appropriate techniques to achieve the set objective.

2 – Budget and neighborhood: details which, if overlooked, can make life unbearable

The costs of expanding a home vary ad infinitum, depending on the type, size and materials used. Generally, horizontal extensions are less expensive than extensions. In this matter, only one rule: the budget must be defined precisely in advance, in accordance with the expertise of professionals, and respected at each phase of the site.

Modifications to buildings are a common source of conflict (including in court) with the neighborhood. It is important not to underestimate them. Explaining the project to those around you, or even asking for advice, is a reassuring process that avoids frustration due to surprise. In all cases, be sure to respect scrupulously national and local town planning rules.

3 – Non-compliance with town planning rules: no authorization to build

The town planning rules define, among other things, the hors d’oeuvre and habitable surfaces, admissible on your land, the indications or obligations in terms of colors, materials or elevation of buildings. This is really the first administrative approach to be undertaken to find out the type of expansion authorized. For example, a plot of 500 m², located in an area, can admit a floor area of ​​800 m² (which implies a floor), while at some distance, the buildable area is limited to 200 m² ( or less), for the same land area. This legal framework is defined by each municipality, in accordance with national regulations, depending on the local particularities of the territory and / or the political options of elected officials.

These rights and obligations are contained in the Local Urban Plan (ENJOYED), available for consultation in the town hall of the place of construction and sometimes online. Beyond the regulations in terms of town planning, the PLU lists all municipal provisions, present or in the process of becoming, in terms of collective accommodation or the environment. You will also find, in the appendix, the water network diagrams and a plan of 4 types of zones (U, constructible, AT to urbanize, NOT green areas and AT agricultural area).

This document, initially off-putting, is truly worth the effort to read. Also make sure you have special rules, if you are located less than 500 meters from a heritage site remarkable. Special rules are applied to protected areas regardless of the built surface.

4 – Building without authorization: financial and legal sanctions

The expansion works of less than 5 m² of surface of floor and up to 12 m height, do not require no authorization administrative (except in protected areas). Beyond that, the filing of a prior declaration of work or a building permit is required.

AT from 1er january 2012, the extensions located in urban areas subject to a PLU (or any other town plan) and a floor area equal to or less than 40 m² are exempt from building permits.

Without urban plan, 2 regimes apply:

  • extensions included between 5 and 20 m² are subject to Statement
  • extensions of more than 20 m² fall under building permit.

The “prior declaration of work”, accompanied by its appendices, must be submitted to the town planning department of the town hall concerned, by hand or by registered mail with return receipt. Municipalities over 3,500 inhabitants, allow digital filing.

The extensions of houses of more than 40 m² are all subject to the filing of a building permit.

Failure to comply with these procedures leads to administrative interruption of the work, fines (up to € 6,000 / m² of built area), prison sentences (in the event of a repeat offense) or final stage, demolition of the structure.

5 – Non-recourse to an architect: risks of refusal of the building permit

Since March 1, 1977, regulations set at 150 m², the “compulsory threshold for recourse to an architect for physical persons which build or modify constructions “(decree n ° 2016-1738 of December 14, 2016). For house extensions, this threshold is for the total surface area. after expansion. For constructions for agricultural use, the limit is increased to 800 m² for buildings and 2000 m² for greenhouses less than 4 m high.

Outside this framework, building permit applications without the claw of an architect, are simply not examined.

6 – Non-declaration of completion of work: administrative and fiscal penalties

The client is obliged to submit to the town hall, as soon as the construction is completed, a file in 3 copies, containing the forms and legal supporting documents proving the successful completion of the work in accordance with the rules of the art and according to the regulations. in force. This file is submitted to the control of the town planning department which has a 3 m delayois to instruct him (or 5 months for the protected areas or close to historical monuments). After these deadlines, the file is deemed approved. You can therefore claim, within two weeks, to the mayor, the Declaration Certifying the Completion and Compliance of the Works (DAACT) by registered letter with acknowledgment of receipt.

Without prejudging criminal and financial penalties, the absence of DAACT deprives you of:

  • of the faculty of to sell your property as a habitable building,
  • protection of decennial and structural damage insurance, the coverage period for which starts on the day it is obtained.

The non-conformity of the work suspends the investigation period and may lead to a formal notice of fault correction, upon removal of a modifying building permit or in extreme cases, at the destruction of the work.

And don’t forget: the owner has 90 days, after obtaining the DAACT, to declare the end of the work to finance administration (art-1406 of the General Tax Code).

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