Souplex in a Condominium: Rules to Follow Before Getting Started

What Is a Souplex?

A souplex is a duplex-style apartment spread over two levels, with one level located in the basement. In most cases, it involves a ground-floor apartment connected to a cellar or lower-level space owned by the same person.

The transformation generally requires the creation of a stairwell opening (typically 1 to 2 meters wide) to connect the two levels. The basement can then be converted into a living area, bedroom, office, or leisure space, provided that applicable building and safety standards are respected.

This configuration allows owners to enhance an existing property and optimize the use of previously underutilized spaces.

In a Condominium: Obtaining Co-Ownership Approval Is Essential

Before starting any work, obtaining approval from the condominium association is mandatory. Creating a souplex usually involves modifying common areas (floors, load-bearing walls, structural openings), which requires a vote at the general assembly.

The decision must be adopted by an absolute majority of co-owners. In addition, the project must comply with the condominium regulations. To minimize the risk of disputes, it is strongly recommended that the transformation does not alter the ownership shares (tantièmes).

Required Planning Permissions

Beyond condominium approval, administrative formalities may also apply:

• Between 5 and 20 sqm of newly created space
  → A prior declaration of works must be filed with the local planning authority (city hall).

• More than 20 sqm created or modification of the exterior appearance
  → A building permit must be obtained before works begin.

It is also essential to consult the Flood Risk Prevention Plan (PPRI). In certain municipalities, the creation of basement dwellings is strictly prohibited, particularly in areas exposed to flood risks.

Technical and Costly Works

Creating a souplex represents a significant financial investment. On average, renovation costs range between €1,600 and €1,800 per square meter, depending on the complexity of the project.

Key technical constraints include:

• thermal and acoustic insulation of walls and ceilings,

• potential loss of ceiling height due to technical installations,

• installation of mechanical ventilation (VMC), essential for proper air quality,

• in some cases, major structural works (reinforcement, drainage, waterproofing).

It is important to note that, in the context of a rental, basement square meters are not legally considered “habitable surface area,” even if used as such.

An Attractive Product on the Rental Market

Despite these constraints, souplex properties are highly appealing to certain tenants. They are perceived as atypical homes offering more space at a price often more attractive than an equivalent traditional apartment.

This uniqueness generally results in very low vacancy rates, particularly in tight urban markets. For investors, a souplex can therefore be an interesting value-enhancement strategy — provided that all regulatory requirements have been carefully anticipated.