Cost to finish a basement in Luxembourg (2026)

A €42 000 HT project at TVA 17 % invoices at €49 140 TTC; on a primary-residence conversion qualifying for 3 %, the all-in figure drops to €43 260 TTC — a €5 880 saving.

Format drivers:

• Starting moisture — a dry cellar (hygrometer under 55 %) skips exterior drainage; a semi-dry cellar adds €3 500–€7 500 for interior tanking; a wet cellar requires exterior French drain and slab injection, €18 000–€35 000
• Ceiling height — below 2,30 m, either lower the slab (€18 000–€35 000) or raise the ground floor (€12 000–€22 000); both require structural engineering
• Egress compliance — habitable rooms need a natural light source at 1/8 of floor area and a secondary egress
• Utilities split — a separate electrical meter for rental use adds €1 800–€2 600; a separate water meter adds €900–€1 400

A Luxembourg cellar counted on the cadastre as non-habitable cannot become habitable without an explicit commune authorisation. The distinction matters for resale value, insurance and the legal rental area declared on the bail.

The four permit categories in practice:

• Déclaration de travaux — for finish-only works that do not change the cellar's legal status (painting, flooring, storage shelves). Free or under €150 processing, one to three weeks.
• Autorisation de construire simplifiée — for minor structural work (adding a partition, upgrading electrical subpanel) without change of use. Fees €150–€500, two to six weeks.
• Autorisation de construire avec changement d'affectation — mandatory for converting a cellar to a habitable room. Requires architect-stamped plans, structural engineer statement on ceiling and load, ventilation study. Fees €500–€2 500 depending on commune and surface. Processing three to five months.
• Autorisation pour logement séparé — for carving a rental studio from the basement. Requires separate egress, compliance with Règlement Grand-Ducal on minimum habitability (natural light, ventilation, ceiling height 2,30 m), and a fire-compartmentation line. Fees €1 000–€3 500, processing four to seven months.

What commune officers check on-site:

• Ceiling height measured floor to finished ceiling
• Natural light aperture at 1/8 of floor surface minimum
• Two independent egress routes for a habitable studio
• Fire-rated door separating the studio from the main boiler room
• Radon ventilation measurement (several Luxembourg communes in the south exceed 300 Bq/m³)

Filing the permit before works start is not optional — a post-hoc régularisation is €3 000–€8 000 more and risks refusal, which forces demolition.

Basement-finish scope varies more than any other renovation because the starting state swings so much. A written scope document protects both parties.

Included in a typical €45 000 quote for a 60 m² office + storage conversion:

• Site survey with hygrometer and radon reading
• Interior tanking coat on all below-grade walls
• 80 mm rigid insulation on walls and floor, vapour barrier tight to slab
• Metal-stud partitions, 12,5 mm plasterboard, acoustic mineral wool between rooms
• Suspended ceiling with integrated LED downlights
• Engineered vinyl flooring or sealed polished concrete
• Electrical subpanel with 8 to 12 circuits, declared and signed off by a registered electrician
• Mechanical ventilation (VMC double-flow) sized for humidity control
• Paint, door frames, 60-minute fire-rated door to the rest of the house
• Architect coordination fee and commune permit filing

Usually quoted separately:

• Structural engineering for height correction — €2 500–€6 000
• Slab lowering and underpinning — €18 000–€35 000
• Shower room rough-in and finish — €6 500–€11 000
• Kitchenette plumbing and cabinetry — €4 500–€9 000
• Separate electrical and water meters — €2 700–€4 000
• Radon mitigation system — €1 800–€3 500

Red flags:

• No hygrometer or radon reading before quoting — blind scope, expensive surprise
• Waterproofing by paint only on a visibly damp wall — cosmetic cover-up, mould returns in 18 months
• No mention of Règlement sur les bâtisses or commune permit — unlicensed scope
• Electrical work by the general builder rather than a declared electrician — not signed off, insurance void
• One-year warranty only — 10-year décennale is the legal standard for structure

Basement works on a primary residence at least 10 years old at the start of the works qualify for the 3 % super-reduced TVA rate through an AED application filed before works begin. The distinction matters because 14 percentage points on a €45 000 net project equals €6 300 in cashflow.

Rate decision tree:

• Main residence at least 10 years old, habitable conversion: 3 % on the full scope including materials and labour, capped at €50 000 of TVA advantage per dwelling
• Main residence at least 10 years old, utility conversion (storage): 3 % generally accessible on the same terms
• Second home, pied-à-terre: 17 % standard, no derogation
• Rental studio carve-out: creating a separate studio counts as creation, which needs the owner's own main residence — a studio let to a tenant stays at 17 %
• Property under 10 years old: 17 % — unless the works take place within 5 years of purchase

Practical invoicing:

• Contractor applies 3 % directly on the invoice once the AED approval number is issued
• The approval reference, owner's address and cadastre reference appear on the invoice line
• TVA credit cap tracking is the owner's responsibility — keep every invoice in the same dossier
• Deposits (acompte) follow the same rate as the final invoice — do not accept 17 % on a deposit against 3 % on the balance

Rate comparison on a €45 000 net project:

File the application early — the AED approval takes 4 to 8 weeks and must be in place before the works start for 3 % on the invoice; otherwise 17 % is invoiced and the difference is reclaimed.

A basement conversion quote without a shared scope document is a coin-flip. Three firms will price three different projects unless the brief is explicit.

The six lines to compare on equal terms:

• Moisture starting state. Insist each quote states the hygrometer reading from the site visit and the assumed waterproofing method (paint-only, cementitious tanking, interior membrane, exterior French drain). Anything less is guesswork.
• Ceiling height solution. If the measured height is below 2,30 m, the quote must state either slab lowering or floor raising, with structural engineering as a separate line.
• Permit strategy. Habitable conversion requires Autorisation de construire avec changement d'affectation. If a quote skips this line, ask why.
• Utility split. Is a separate electricity meter in scope? A separate water meter? For a rental studio, both are non-negotiable.
• Finishing level. Plasterboard-to-paint, tiled floor, engineered vinyl, polished concrete — each adds or subtracts €40–€90/m². Pick one target level and specify it.
• TVA rate line. Confirm the rate applied on the quote and, if 3 %, that the AED application will be filed by the firm or the client.

A clean briefing pack:

• Measured floor plan with existing partitions and load-bearing walls
• Starting hygrometer reading at two points
• Photos of any visible cracks, efflorescence, stains
• Target use per area (office, storage, shower, kitchenette)
• Preferred ceiling height (2,30 m standard, 2,40 m premium)
• TVA position and AED reference if filed

Three quotes on the same pack stay within ±12 %. A wider spread means someone is omitting a structural line.