Actualité - Horizontal house extension: procedures, constraints and budget in 2026

A horizontal extension consists of enlarging a single-storey house under construction by extending the existing structure onto the available land, as opposed to raising the roof, which gains surface area in height. In France, the administrative procedures depend directly on the surface area created: a simple prior declaration is sufficient up to 20 or 40 m² depending on the zone of the local urban development plan (PLU), a building permit becomes mandatory beyond that, and the use of an architect is required if the total floor area of the property exceeds 150 m². The budget varies significantly depending on the chosen material, from €900 to more than €2,500 per m² depending on whether it is a veranda, a timber frame, or a breeze-block extension.

Key takeaways

A horizontal extension enlarges the house at ground level, without touching the existing structure, unlike raising the roof, which alters the roofing and the load-bearing foundations.
The administrative regime depends on the surface area created: prior declaration up to 20-40 m² depending on the PLU, building permit beyond that, and an architect mandatory if the total floor area exceeds 150 m².
The average budget ranges from around €900 per m² for a simple veranda to more than €2,200 per m² for a high-end masonry extension.
A well-designed extension increases the value of the property but also raises the development tax (taxe d'aménagement) and the property tax (taxe foncière), two factors to anticipate when calculating profitability.

Why choose a horizontal extension rather than moving house

Enlarging one's house rather than moving reflects a financial and emotional logic well known to French homeowners. Moving involves notary fees, agency fees, a new loan potentially at a less favourable rate, and the loss of a cherished living environment. A horizontal extension allows homeowners to keep their neighbourhood, schools and habits, while meeting a need for space that has become pressing.

The most common motivations are the arrival of an additional child, the fitting out of an office for remote working, the accommodation of an elderly parent, or the creation of an independent master suite. In a context where loan rates remain high and the resale market is sometimes slow, extending one's existing house often represents a faster and more financially controlled alternative than a new purchase.

Horizontal extension or raising the roof: what are the differences

Raising the roof consists of adding a storey above the existing house, relying on the load-bearing walls and foundations already in place. It is preferred when the plot does not have sufficient buildable surface area, but it often requires costly structural reinforcement and the total or partial removal of the roof.

A horizontal extension, on the other hand, is built at ground level, extending from a façade. It requires its own foundations and a carefully executed connection to the existing structure, but avoids the load constraints on the original building. It is generally simpler to carry out technically, provided the plot is large enough and the distance rules from the property boundaries set by the PLU are respected.

Which planning procedures depending on the surface area created

Prior declaration of works

For an extension creating between 5 and 20 m² of floor area or ground footprint, a prior declaration of works is sufficient in most municipalities. This threshold is raised to 40 m² in urban areas covered by a PLU, provided that the total surface area of the construction after works does not exceed 150 m². The application is processed by the town hall within a period of approximately one month.

Building permit

Beyond these thresholds, a building permit becomes mandatory. This is also the case as soon as the extension, even a modest one, brings the total floor area of the house beyond 150 m², a threshold that also triggers the obligation to use an architect. The processing time for a building permit for an individual house is generally two to three months.

Mandatory use of an architect

As soon as the total floor area of the building, including the extension, exceeds 150 m², the use of an architect is required by law to draw up the architectural project submitted to the town hall. This threshold concerns the overall surface area of the house after works, and not only the surface area created by the extension, which often surprises owners of already spacious houses.

From 5 to 20 m² (or 40 m² in a PLU zone): prior declaration of works.
Beyond these thresholds: building permit mandatory.
Total floor area exceeding 150 m²: use of an architect required.

Technical constraints to anticipate

A horizontal extension cannot be improvised from a technical standpoint. Several points of vigilance determine the soundness and compliance of the project.

The foundations must be adapted to the nature of the soil and deep enough to avoid any differential settlement with the existing structure, a classic source of cracks.
Connection to the existing building concerns the structure, but also the water, electricity and sometimes sewage networks, which must be extended or reinforced according to the new living area.
The distance from the property boundaries is governed by the local zoning plan (PLU) and the Civil Code, which generally impose a minimum setback in the absence of a party wall, under penalty of neighbour disputes.
The thermal insulation of the extension must comply with current environmental regulations, which influences the choice of materials and the final budget.

A preliminary soil survey and a structural study are strongly recommended before any final costing, particularly on clay or sloping land.

Average budget per m² depending on the type of extension

The cost of a horizontal extension varies considerably depending on the construction technique chosen, the desired finish and the region. Below are the orders of magnitude commonly observed in 2026 for a turnkey extension, excluding high-end interior fittings.

Conservatory: from €900 to €1,800 per m², depending on the structural materials and the glazing chosen.
Timber-frame extension: from €1,500 to €2,200 per m², a solution appreciated for its fast build time and thermal performance.
Breeze-block or traditional masonry extension: from €1,600 to €2,500 per m², often preferred for a perfect aesthetic continuity with the existing house.

To this construction budget must be added architect's fees where required, soil survey costs, planning taxes and any landscaping work made necessary by the works. It is wise to allow a margin of 10 to 15% for unforeseen building costs.

Impact on the value of the property and on taxation

A well-integrated horizontal extension, in architectural harmony with the original house, generally increases the value of the property beyond the mere cost of the works, particularly when it creates a sought-after room such as a master suite or a separate office. It can also make the property more attractive to a family looking for space, an increasingly decisive criterion in the market.

Any created surface area of more than 5 m² triggers, however, the payment of the taxe d'aménagement (development tax), calculated on the basis of a flat-rate value per m² set each year by decree, multiplied by a municipal and departmental rate that varies by area. The extension also increases the taxable surface area used to calculate the property tax (taxe foncière), which must be factored into the medium-term financing plan. Before launching an extension project, it is wise to compare its actual cost to the expected capital gain on resale, an analysis for which an online property valuation provides a quick initial benchmark figure.

Conclusion

Horizontal extension remains one of the most effective solutions for gaining living space without moving house, provided that the administrative procedures linked to the created surface area and the total floor area of the property are precisely anticipated. The choice of material, the nature of the soil and the distance from the property boundaries strongly determine the final budget, which must include not only the construction cost but also the development tax (taxe d'aménagement) and the change in property tax (taxe foncière).

Before getting started, it is recommended to have the project precisely costed by qualified professionals and to check its actual impact on the value of the property. An Optimhome estate agent knows the local market prices and can help assess the financial relevance of an extension before starting work, or provide support for a future sale once the project has been completed.

FAQ

What extension surface area can be created without a building permit?

An extension creating up to 20 m² of floor area or ground footprint is subject to a simple prior declaration of works. This threshold rises to 40 m² in areas covered by a local zoning plan (PLU), provided that the total surface area of the building after the works does not exceed 150 m². Beyond this, a building permit becomes mandatory.

Is an architect mandatory for an extension?

Using an architect is only mandatory if the total floor area of the house, including the extension, exceeds 150 m² after works. Below this threshold, the owner can file the application themselves, even though it is still advisable to be assisted by a professional for the design.

How much does a 20 m² extension cost on average?

Depending on the construction technique, a 20 m² extension generally costs between €18,000 and €50,000 in 2026, excluding architect's fees and taxes. A conservatory remains the most economical option, while a high-end masonry extension falls at the upper end of the range.

Does a horizontal extension automatically increase the property tax (taxe foncière)?

Yes, any living space created is included in the calculation base of the taxe foncière, which mechanically leads to an increase, generally proportional to the area added. The taxe d'aménagement, for its part, is payable only once, shortly after completion of the works.

What distance must be respected from the neighbour for an extension?

The minimum distance depends on the rules of the local PLU (local urban development plan) and, failing that, on the Code civil, which generally requires a setback of several metres in the absence of a party wall. It is essential to check the commune's planning regulations before drawing up the project, as these distances vary considerably from one zone to another.

Is a horizontal extension always profitable at resale?

Not systematically: profitability depends on the quality of architectural integration, the local market and the type of room created. A well-designed master suite or office generally adds value to the property, whereas a poorly proportioned extension can, on the contrary, harm the overall harmony of the house. An online property valuation before works makes it possible to gauge the potential difference in value, and looking at Optimhome property listings helps compare similar properties already extended in the area.