Five kinds of data that change what a tenant pays or a building is worth

German law lets a landlord ask a sitting tenant to agree to a higher rent, up to the rent customary in the locality and within limits on frequency and size, and the request must give reasons: a rent index, a rent database, an expert's opinion or at least three comparable flats. My team prepares the data behind about 52,000 such letters a year. About 21,000 of them go to flats in places where no usable index covers the flat, so they rely on comparison flats from our own portfolio. Since 2023 those flats are chosen by a published rule: same district, construction-year band, floor area within a fifth either way, comparable bathroom and heating, chosen nearest the median rent rather than the top. We also supply complete, unselected rent data to the surveys behind the qualified indices of the towns where we own most, because a landlord that shapes an index should not be able to choose what it contributes.

Heating and hot water in our 8,900 centrally heated buildings are billed largely by measured consumption, through heat-cost allocators on radiators and meters read by metering service providers. My team reconciles fuel invoices, readings and building data so that annual statements arrive within the twelve months the law allows, and earlier where we can. Since the 2021 amendment of the Heating Costs Ordinance we owe monthly consumption information wherever devices can be read remotely; we rebuilt that message in 2024 to show euros, the same month last year and similar flats in the building. Since 2023 the carbon price on heating fuels has been split between landlord and tenant according to a building's emissions per square metre, which turned heated living area into a figure with money attached. Where our area records were unresolved, we classified buildings in the tenant's favour.

Most of our 11,600 buildings were built between 1950 and 1979, and fewer than a fifth have reliable as-built drawings. A model of a building is only as good as what someone has actually observed inside it, so I resist portfolio-wide twins populated by assumptions. Our condition model covers the 1,840 buildings in a €460 million programme to replace drinking-water and heating risers between 2024 and 2031. It holds camera inspections of at least one riser per stairwell, pressure tests, results of the legionella testing German rules require for larger hot-water systems in rented buildings, caretakers' leak reports and insurance claims, each with its date and method. The remaining buildings are planned on construction-year assumptions, and we say so. In 2026 we are extending the model to 1,100 buildings with façade or heating work due within five years.

A landlord holds some of the most personal records a household produces: pay slips, identity papers, credit-agency reports, payment histories and caretakers' notes that can touch on health or family circumstances. About 61,000 people apply for our flats each year, and most are not offered one. Under the GDPR we may keep personal data only as long as we need it for a stated purpose. In 2023 we found about 4.1 million documents we no longer needed, some from 2009, and by December 2024 we had deleted 3.4 million. We now ask for pay slips only from the final shortlist for a flat, delete applicants' documents six months after a letting, and replace free-text caretaker notes with structured fields. Our data-protection officer advises independently; my team runs the processes, including access requests, which we now answer in nine days on average.

Like other listed property companies reporting under IFRS, we carry investment property at fair value, estimated each December by external valuers, and a one per cent movement on €19.6 billion is close to €200 million through the income statement. The valuers' judgement is theirs; the evidence is mostly ours. My team prepares the data tape: rent roll, vacancy, lease data, floor areas, modernisation history and, since 2023, our condition evidence with its date and source, together with the share of buildings for which we hold none. I present the controls over that tape to the audit committee. The same building records now feed our sustainability reporting under EU rules whose scope and timetable were still being revised in 2025, so I insist that a floor area or an energy figure means the same thing in the valuation, the heating statement and the annual report.