Preface: Why Germany, and Why One Family

This book is first of all a demonstration of what modelling legislation with Arxo can do. Its model is an independent reading of the published German texts, built for teaching; it does not claim legal accuracy or completeness, it is not an implementation by or for the BMWSB or any Wohngeldbehörde, and its answers are not legal advice. For a real claim, rely on the official texts and on the Wohngeldbehörde.

Germany is a good place to learn law as code. Federal law is free to read on gesetze-im-internet.de, and German statutes are free to reuse (§ 5 UrhG). Its civil code gave the world a general part and a systematic order that many codes still follow, and its Rechtsinformatik — legal informatics — has been a field since 1970. And for the benefit this book follows, the federal ministry responsible for housing, the BMWSB, publishes worked examples of its own formula: the incomes, rents and amounts they print are an official check on the model’s numbers. GETTSIM models German taxes and transfers in the open, and Catala, from Inria, showed that a statute can be written as code beside its own text. This book stands on that work, and its acknowledgements say how.

The book follows one benefit, Wohngeld — Germany’s housing benefit — and one family: the Vogts, a couple with two children in a rented flat in Weimar. The names are ours; their facts and numbers are those of the ministry’s Rechenbeispiel 7 for 1 January 2025. Other households join where a chapter needs them: a woman whose job centre already pays her housing costs, a neighbour who owns her flat, a single man in Ludwigshafen am Rhein for the final exercise. Every answer in the book comes from a run of an executable model of the published rules, and every answer can be traced to the section, the edition and the facts behind it.

That model is an independent Arxo model built from the published texts of the WoGG, the WoGV and the EStG. It is not an implementation by or for the BMWSB or any Wohngeldbehörde, and nothing in this book describes how a particular office processes a file.

What the book is for

The model answers more than “how much”. A housing-benefit file raises questions a calculator does not: which edition of the annexes applies on the date of the question, what happens on the day the values change, what the answer is when a fact is missing or two records disagree, which questions the Act leaves to the authority, which facts were decided by another authority, and how a saved answer is checked again a year later. Each chapter takes one of these questions and shows what the rules establish, what remains open, and who must act next.

The book is written for the people who build and run such systems together: lawyers and staff of housing-benefit authorities, programme analysts, and developers. You do not need to know Arxo, and German helps but is not required: every German term is glossed in English where it first appears, the sections are quoted in German with an English gloss, and a glossary at the end lists the terms.

What the book is not

This is not legal advice, and it is not a guide to anyone’s entitlement. The households are fictional. The amounts are answers to dated questions about stated facts: a different date, a different fact, or a later edition of a text can give a different answer, and the book shows exactly how. The ministry’s worked examples are examples; as its own brochure says, they establish no legal claim.

A short book

This volume is shorter than the first in the series: one benefit, one family, ten chapters and a final exercise — what a reader needs to follow a decision from the gazette to the claim. The procedure after the amount — the Bescheid, duties to cooperate and deadlines — and the bill for 2027 are planned for a later edition.

Dates

Every question in this book is asked at a legal date — for the Vogts, 1 January 2025, and 31 December 2024 when the chapter compares the two editions — against the texts and annex values in force on that day. Where a chapter shows a recorded run, it gives the date of the recording as well: most answers were recorded on 12 October 2026. The two are different: a question about 1 January 2025 can be asked again years later and must give the same answer from the same texts.

An online edition

The same text is also an online course. The book and the course are one text in two forms; corrections reach both.