A contingency is not a rounding error added at the end. It is sized against the three categories where scope is discovered rather than specified, and in an apartment one of them is not yours to control.
The three categories that produce overruns
Services. Electrical installation, plumbing, heating and ventilation. In an older property nobody can specify these accurately until walls and floors are opened, which is why they are the largest source of surprise.
Substrate. What is behind the finish: the state of a floor under a covering, a wall under plaster, a ceiling above a suspended one.
The building. In a copropriété anything touching the structure, the facade, the common systems or the extraction requires consent and frequently the general assembly's authorisation, and assemblies meet once a year.
Finishes rarely overrun, because they are chosen from a catalogue with a price. It is the three above that move.
Sizing it against documents rather than a rule
A generic percentage tells you nothing about a specific property. Four documents do.
The dates of the last replacement of the electrical installation, the plumbing, the heating and the windows, with evidence. Where all four are documented and recent, the services risk is low. Where nobody can answer, treat that as unfavourable.
The minutes of the last three general assemblies, since works voted and not yet called for are an existing liability rather than a risk and should be provisioned at full value.
The reconciled charges for two completed years, not the monthly provision.
And the fonds de travaux, compulsory in every Luxembourg co-ownership since 1 August 2023 under the law of 30 June 2022, with the annual contribution voted at the assembly but not below a floor fixed by grand-ducal regulation per square metre of energy reference surface, graded by insulation class.
What you may not do without permission
This is where apartment renovations differ most from houses, and where budgets are lost to timing rather than to cost.
The structure, the facade, the roof, the common systems and usually the window joinery are common parts under the amended law of 16 May 1975. Improvement works require an absolute majority of all millièmes, present or not, which in a building with absentee owners is genuinely hard to assemble.
So before budgeting anything, separate the works into what is inside your lot and what touches the building. The second category needs a decision from people who are not you, on a timetable that is not yours.
The order that reduces the risk
Open the services first. An electrician and a plumber inspecting before you commit to finishes will tell you more about the real cost than any estimate produced from a floor plan.
Where that is not possible before purchase, make it the first work done rather than the last. Discovering a rewiring requirement after the floor is laid is how a contingency becomes a second loan.
Keep it separate from the reserve
The contingency covers the renovation. The reserve covers the building: a special call for funds voted by the general assembly, which binds every owner and which the syndic is obliged to pursue on its own initiative.
Holding one figure for both is how owners find themselves renegotiating financing while a property is open, which is the worst possible moment. Write them as separate lines so that deleting one is a decision rather than an accident.
What the bank sees
Lending follows CSSF Regulation 20-08 of 3 December 2020, with a general loan-to-value principle of 80%, first-time buyers of a principal residence financeable up to 100% of the price, and buy-to-let at 80%. Acquisition costs are normally excluded from financing.
Lenders assess remaining disposable income alongside the debt-service ratio commonly cited around 40%, which is banking practice rather than a legal rule, and apply a stress test so the loan is sized against a rate materially higher than the one offered.
A file that shows a visible contingency after completion is a stronger file rather than merely a safer one, and it is visible on the statements you are already providing.
The energy dimension
Energy class now affects financing as well as comfort. Properties classed A to C generally attract the best lending conditions; F to H typically trigger stricter analysis, with works folded into the financing plan rather than treated as optional.
That cuts both ways. A poor class is a negotiating argument and a real cost, and the works that improve it frequently touch the envelope, which is a common part requiring the assembly's authorisation rather than yours.
What to do when the contingency is not used
Hold it until you have been through one full charges reconciliation and one general assembly. That is when the building's real cost and its immediate intentions both become visible.
Releasing it the week the work finishes is premature, because the categories that produce surprises in an apartment are frequently the ones on the building's timetable rather than yours.
Three questions we are asked
Is a percentage ever useful? As a sanity check on a fully specified job with recent services, yes. On an older property with undocumented installations, it is a guess dressed as a method.
Can contingency be financed? Sometimes, and it is generally better to hold it than to borrow it, because a facility can be reduced or withdrawn and is typically at a higher rate.
Should a poor building reduce my offer? Documented liabilities in the assembly minutes are a legitimate argument. A general impression that the building looks tired is not.
A worked sequence
Before the offer: get the four dates on the services with evidence, the minutes of the last three assemblies, two years of reconciled charges and the works fund balance and annual contribution.
Before committing to finishes: have an electrician and a plumber inspect. Where that is impossible before purchase, make it the first work done rather than the last.
Before signing any quotation: separate the works into what sits inside your lot and what touches the building, because the second category needs a decision from the general assembly, which meets once a year.
After completion: hold the reserve until one full charges reconciliation and one assembly have passed. That is when the building's real cost becomes visible.
Three questions we are asked
Should the contingency be spent if the work goes well? Not immediately. Hold it through one charges reconciliation and one general assembly, since in an apartment the surprises frequently arrive on the building's timetable rather than yours.
Who decides what is common and what is private? The acte de base and the règlement de copropriété. Read both before planning anything that touches a wall, a facade or a service.
Editorial note: This is a framework for a housing decision, not advice on a specific property. Communal rules, building regulations and market practice in Luxembourg change. Verify with your commune, your lease or règlement de copropriété, and guichet.lu before relying on any of the above.