Why Two 80 m² Flats in Luxembourg Can Be Different Sizes

The national standard: ILNAS 101:2016

Luxembourg's national standardisation body, ILNAS (Institut luxembourgeois de la normalisation, de l'accréditation, de la sécurité et qualité des produits et services), published a standard in 2016 specifically to give the market a uniform way of calculating a home's surface habitable (living area), replacing the patchwork of methods agents and architects had used previously.

Under ILNAS 101:2016, the surface habitable is broadly the sum of the floor areas of rooms that genuinely function as living space: bedrooms, living rooms, kitchens, bathrooms, toilets, and internal circulation space such as hallways and stairs, measured from the interior face of the walls, excluding the thickness of walls and load-bearing structural elements themselves.

What the standard excludes

The standard is precise about what does not count toward the living area, and this is exactly where two listings can diverge:

• Terraces, loggias and balconies, even when they are usable outdoor space attached to the flat.
• Unheated verandas.
• Cellars and garages, regardless of size.
• Any part of a room where the ceiling height is below 2.00 metres. This is the rule that most often trips up sellers of homes with sloped ceilings: an attic room with a generous floor footprint can lose a substantial share of its apparent area once the low-height strip under the eaves is excluded.

Attics and mezzanines

An attic or mezzanine counts toward the living area only if it is genuinely usable: properly floored, with adequate headroom, reasonable access (not a ladder through a hatch), and free of exposed structural framing that makes the space impractical to furnish. An unconverted, unfloored attic with restricted access contributes nothing to the official living area figure, even if it is technically walkable.

Is the standard actually mandatory?

Here is the detail that surprises most buyers: the ILNAS standard is not legally binding. It is a national standard, meaning it exists as a recognised, consistent methodology that professionals are encouraged to use, but nothing compels an agent, a developer or a private seller to apply it rather than another method, or to specify which method they used at all. In the absence of any other agreement between the parties, a court asked to resolve a dispute over a stated surface would likely default to the ILNAS methodology as the closest thing to an accepted market standard, but that is a fallback for disputes, not a guarantee built into every listing you see.

The practical consequence is that a "surface habitable" figure on a portal listing could, depending on who produced it, include or exclude a balcony, a sloped-ceiling attic, or a heated veranda, without any indication of which convention was used. Two flats billed as identical sizes can genuinely differ in usable space once you understand this.

New builds: a different measure entirely

If you are buying off-plan, under the VEFA (vente en état futur d'achèvement) system, the surface quoted in the reservation contract and the deed is often not the ILNAS living area at all, but a surface construite brute or a similarly defined gross or "utile" measure, which can include elements the ILNAS standard would exclude, calculated against different height thresholds (a commonly cited difference is a 1.80 metre versus a 2.00 metre ceiling-height cutoff between the two conventions). Our guide to buying off-plan in Luxembourg covers the guarantees and payment schedule that come with a VEFA purchase; on the surface question specifically, the rule of thumb is simple: never compare a VEFA-quoted surface directly against an ILNAS-based surface habitable for a resale property, since they are not measuring the same thing, and the difference can run to several square metres on an average flat.

VEFA contracts also typically include a tolerance margin, commonly around one-twentieth (5%) of the stated surface, within which a difference between the contracted and the actually delivered surface does not automatically give rise to a price adjustment or a right to walk away. Read this clause in the reservation contract, not just the headline surface figure, since it defines how much variation you are contractually accepting in advance.

What to actually ask, at a viewing or before signing

• Which standard was used to produce the quoted surface: ILNAS 101:2016, a gross construction surface, or something else entirely?
• Is a balcony, terrace or veranda area included in the headline figure, or listed separately?
• For a sloped-ceiling room, has the sub-2-metre portion been excluded, and what is the resulting usable area?
• For a co-ownership, does the quoted surface match the surface recorded in the co-ownership deed (the base for calculating your share of the quote-parts and, therefore, of the charges)?
• For VEFA, what tolerance margin applies, and what happens contractually if the delivered surface falls outside it?

If a seller or developer cannot say which convention was used, ask for the room-by-room breakdown and check it against the plan yourself; a genuine ILNAS calculation should hold up to that scrutiny, and a loosely estimated figure usually will not.

Why this matters for price comparisons

Price per square metre is one of the most common ways buyers and agents sanity-check an asking price, as explained in our guide to how property is valued. That comparison is only meaningful if the square metre figures on both sides of it were measured the same way. A flat that looks expensive per square metre using an ILNAS figure might look ordinary once you notice a competing listing quietly included its balcony and a generous sloped attic in its own "surface habitable."

Our take

Do not take a headline square metre figure at face value, on either a resale flat or a new build reservation. Ask which standard produced it, and, where it matters to your decision, have the space independently checked, particularly for an attic conversion, a flat with a large balcony, or any off-plan purchase where the contracted surface and the tolerance clause deserve a careful read before you sign. If you want a second opinion on a listing's stated surface before you make an offer, contact us, or browse our current listings, where we work from verified plans rather than headline figures alone.

This article describes general rules and market practice in Luxembourg and is not legal, tax or financial advice. Rules and announced measures change; confirm the position that applies to you with a notary, your bank or a tax adviser before signing.