Lounbrëck counts every frontalier's days worked outside Luxembourg against all three threshold lines — the 34-day tax line that breaks first, and the 25% / 49% social-security lines behind it — and warns you before the tax bill or the CCSS letter arrives. An AI assistant does the logging, the checks, and the explaining.
The problem
Generic HR tools don't count cross-border days. Payroll runs as usual. The rules don't wait.
A frontalier works from home one day a week — plus a few extra days in winter. Nobody counts. At day 35 outside Luxembourg, every one of those days becomes taxable in their residence country — back to 1 January. A fixed limit, the same 34 days for France, Belgium and Germany. It is almost always the first line crossed.
A few weeks later, past 25% of working time, affiliation legally moves from Luxembourg (CCSS) to the residence country — URSSAF, ONSS, or DRV. Contributions keep being paid as if nothing changed. Every month compounds the mismatch: wrong country, wrong rates, wrong employer obligations.
When it surfaces — an audit, an A1 request, a hospital claim — both regimes reassess retroactively. The social-security liability is the employer’s alone: CCSS can reclaim contributions up to five years back, plus penalties, on top of the residence-country tax. Combined exposure runs well into five figures. For one employee.
The fix is unglamorous: count the days, every day. That's what Lounbrëck does.
How it works
A CSV with name, residence country, and start date is enough. No HRIS integration, no IT project — most teams are live in ten minutes.
One click per day, a whole month in one action, or let your AI assistant do it. Lounbrëck recalculates every threshold on every entry.
Warnings at 70%, critical at 90% — by email, Slack, or Teams. Deadlines for A1, CCSS, and ACD filings generate themselves.
The product
Live view · not a screenshot
SAFE → WARNING → CRITICAL → EXCEEDED on every line — the 34-day tax limit and the 25% / 49% social-security ceilings — recalculated on every logged day, per corridor.
A1 renewals, CCSS declarations, and ACD tax cards generate themselves from your employee data.
Tax class under Art. 157ter LIR and routing across CCSS, URSSAF, ONSS, and DRV — versioned, auditable rules.
AI-first
An assistant lives inside the dashboard — it reads your real data, logs days, runs checks, and explains the rules in plain language. And through our MCP server, Claude and any AI agent can operate Lounbrëck directly: seventeen tools, the full regulation reference, your data, your permission.
Lounbrëck Assistant
Under the hood
Every calculation is an independently versioned engine module — auditable, peer-reviewable, updated when treaties change. Read the engine docs
EU REG. 883/2004
Social security router
engine/social-security
ART. 157TER LIR
Tax treaty engine ×3
engine/treaty/lu-{fr,be,de}
TAX 34D · SS 25/49%
Three-line compliance engine
engine/compliance
Pricing
The risk is per frontalier employee, so the price is too. One avoided penalty pays for years of Lounbrëck.
Free
Up to 3 frontaliers
Starter
4–20 frontaliers
Growth
21–100 frontaliers
Enterprise
100+ frontaliers
No setup fees · Cancel anytime · Free up to 3 frontaliers
FAQ
There are two separate limits, and the stricter one is easy to miss. For tax, a frontalier can work up to 34 days a year outside Luxembourg — a fixed day count, now harmonised across France, Belgium and Germany. For social security, the limit is 25% of annual working time, roughly 55 days on a 220-day year. Because 34 days is reached before 25%, the tax line is usually the first one crossed — and it is the one most payroll tools do not track. With an employer opt-in and a valid A1 certificate, the 2023 Framework Agreement raises the social-security ceiling to 49% (the tax limit is unchanged).
It depends which line is crossed. Past the 34-day tax line, the days worked outside Luxembourg become taxable in the employee’s residence country — retroactively for the whole calendar year. Past the 25% social-security line, the regime shifts from CCSS (Luxembourg) to the residence country’s institution — URSSAF (France), ONSS (Belgium) or DRV (Germany) — which requires an A1 certificate. On the social-security side the liability is the employer’s alone: CCSS can reclaim contributions up to five years back, plus administrative penalties, on top of the residence-country tax. Combined exposure for a single employee runs well into five figures.
Lounbrëck supports all three Luxembourg cross-border corridors: LU-FR (Luxembourg–France), LU-BE (Luxembourg–Belgium), and LU-DE (Luxembourg–Germany). Each corridor has its own bilateral tax treaty rules and social security routing logic, all implemented as separate engine modules.
An A1 certificate is an EU document that certifies which country’s social security legislation applies to a worker. For Luxembourg frontaliers it becomes required once social security shifts to the residence country — and, under the 2023 Framework Agreement, an employer can opt in (with an A1) to raise the home-working ceiling from 25% to up to 49% of working time. Lounbrëck tracks each employee’s A1 status and validity dates, and flags when one is required or about to expire.
Each bilateral treaty governs which country taxes which portion of a frontalier’s income. Under Art. 157ter LIR, Luxembourg non-residents may qualify for tax class 2 (assimilation fiscale) when at least 90% of their household income derives from Luxembourg sources. Lounbrëck’s Tax Treaty Engine calculates the applicable tax class for each employee based on marital status and the 90% income test.
No. Lounbrëck is a compliance intelligence layer that sits alongside your existing payroll system and fiduciary. It monitors WFH thresholds, routes social security, and generates filing deadlines in real time — but it does not process payroll or replace professional tax advice. Think of it as the early-warning system your fiduciary wishes you had.
Yes. Lounbrëck acts as a data processor under GDPR, with your organisation as the data controller. All data is stored and processed in the EU (Frankfurt). A Data Processing Agreement (DPA) is available on request. Lounbrëck collects only the minimum data required for compliance calculations — no salary data, no personal income information.
Most teams are fully set up in under 10 minutes. Import your employee list via CSV (name, nationality, residence country, corridor), and Lounbrëck immediately calculates WFH thresholds, SS routing, and upcoming compliance deadlines for every frontalier. The free tier covers up to 3 frontalier employees at no cost.
Free up to 3 frontaliers · No credit card