An employee who commutes to Luxembourg does not bring the practice one more file: they bring a file of a different nature. For most clients, the return is reconstructed after the fact from documents that already exist — payslips, bank statements, certificates. For the cross-border worker, the central item exists only if someone kept it during the year: a day-by-day count of the days worked outside Luxembourg, and enough to prove the others.
The two counters that decide the file
Tax counts days. Tax treaties tax the salary where the work is physically carried out; the tolerance is the exception that allows up to 34 days a year outside Luxembourg without changing anything. The three neighbouring countries arrived at the same figure, but not at the same time: Belgium has applied 34 days since 2022 (24 before), France since 2023 (29 before), Germany since 2024 (19 before). A file covering an older year is therefore read with that year's ceiling, not today's. Once exceeded, the ceiling does not flip everything: only the excess days become taxable in the country of residence — but they trigger a heavier return, and questions.
Social security counts a percentage. From 25 % of working time performed in the country of residence, social security affiliation switches there — the employer included, on the whole salary. The 2023 framework agreement allows up to just under 50 % for habitual remote work, but only on the employer's application. The two counters are independent: a client can be beyond reproach on the 34 days and cross the 25 %, or the other way round. The same calendar serves both readings.
The burden of proof sits with the client
Declaring a Luxembourg salary as exempt in the country of residence means asking for an exception, and whoever asks for the exception proves it. If the authority of the country of residence questions the count, it is not for them to establish that the client was at home on a given Tuesday: it is for the client to establish that they were in Luxembourg. The employer's certificate summarising the days worked is useful but rarely sufficient on its own — it asserts, it does not demonstrate. What convinces is a body of records dated the same day.
What the practice should ask for in January, not in May
The list is short; it is the collection timetable that makes the difference. A record dated the same day is picked up in thirty seconds; the same record, six months later, cannot be made any more.
- the day-by-day annual count: for every day out of the ordinary, the date and the reason — remote work, travel, leave, sick day;
- for days worked in Luxembourg, a reasonable number of proofs of physical presence: badge or clocking records, fuel, till and parking receipts, travel tickets, card payments showing a location;
- for days worked outside Luxembourg, the trace of what they were: assignment order, approved calendar, remote-work agreement;
- the employer's certificate at year end, as a summary that corroborates the count — not as the single item that replaces it.
A client who turns up in May with an empty calendar and a shoebox of undated receipts does not have a late file: they have no file. The most profitable instruction a practice can give a cross-border worker fits in one sentence — a day out of the ordinary, a line the same day, a supporting record attached.
How this relates to the application
That is exactly the register this application keeps: a calendar where the client marks their days as they go, attaches the photographed record, and out of which comes an annual PDF, day by day, with the counters in the footer. For the practice, two read-only doors: a viewing link the client generates and revokes, or a free accountant workspace where every client who has accepted the practice's invitation appears with their counters and their records — with no file exchange, and with nothing the practice can alter. The register stays the client's; the practice reads it the way the tax authority would.
The space itself, and what it shows the practice: accountant space for cross-border clients.