Julian FalkChief Data Officer · Essays & perspective
Rent-setting data5 min read
Property data a tenant can inspect
When a German landlord justifies a rent increase with comparison flats from its own portfolio, the tenant cannot see the hundreds of flats it might have named instead. Fairness depends on a selection rule the tenant could apply and check.
Every year my company sends about 52,000 letters asking tenants to agree to a higher rent. German law lets a landlord raise the rent of an existing tenancy towards the rent customary in the locality for comparable flats, within limits on how often and by how much, and only with the tenant's consent. The letter has to give reasons. Where a town publishes a rent index that covers the flat, the reason is usually the index. Where it does not, the landlord may instead name at least three comparable flats whose rents are as high as the rent requested.
Every figure that changes what a household pays, or what a building is said to be worth, should be one we could explain at that household's front door. If I could not stand in a stairwell and show a tenant how we reached a number, the number is not finished yet.
A German tenant usually learns what last winter's heating cost many months after the winter ended. Remote reading and the carbon-cost split now make it possible, and necessary, to govern heating data for the tenant first and the annual statement second.
External valuers of residential portfolios estimate repair spending from construction year and convention, because owners rarely send them what caretakers and surveyors already know. A data tape without the owner's bad news is a weak control over fair value.
Rented flats whose records my team keeps
196,400
In 11,600 buildings across five German states, December 2025
Net cold rent, 2025
€1.05bn
Average €7.18 per square metre a month; vacancy 2.3 per cent
Rent increases agreed without dispute, 2025
93%
Up from 81 per cent in 2021, after the comparison-flat rule was published
A landlord's data is fair only when the tenant it affects could check it.
I am chief data officer of Haselwerder Wohnen SE, a fictional listed landlord based in Hanover that rents out about 196,400 flats in 11,600 buildings across five German states. Most of our buildings went up between 1950 and 1979 for workers and their families. In 2025 our tenants paid €1.05 billion in net cold rent, an average of €7.18 per square metre a month. I lead 132 people who keep the records behind every rent, heating statement, repair plan and valuation the company produces.
I came to this job from the tenant's side of the table. I spent six years designing surveys for the rent indices towns publish, five producing heating and service-charge statements for a municipal housing company and five checking valuers' assumptions at a mortgage bank. I joined Haselwerder in 2015 to run rent and portfolio analytics and became chief data officer in April 2021.
The decisions that define my work have mostly made a number smaller or slower. We changed how we choose the comparison flats behind rent increases and gave up about €6 million a year of increases we were entitled to request. We settled carbon costs in tenants' favour where our floor-area records were weak, and we deleted millions of documents we should not have kept. I would make each choice again, and these pages show what each one cost.
Rent increases tenants can check
Since April 2023 our comparison-flat letters use a published selection rule and show the tenant the range of comparable rents. Consent without dispute rose from 81 per cent in 2021 to 93 per cent in 2025, and actions for consent fell from about 640 a year to about 140.
Carbon costs settled in the tenant's favour
When the carbon price on heating began to be split between landlords and tenants in 2023, floor-area records disagreed in 1,240 of our 8,900 centrally heated buildings. We placed unresolved buildings in the step better for tenants, at a cost of about €0.9 million, and cut the unresolved list to about 190 by the 2025 statements.
A twin built where the pipes were due
Instead of a €41 million model of the whole portfolio, we built a €9.5 million condition model of the 1,840 buildings in our riser-replacement programme. It moved 212 buildings forward in the sequence, deferred 165, and water-damage claims in those buildings fell from 1,930 in 2022 to 1,420 in 2025.
3.4 million old documents deleted
We deleted copies of identity papers, pay slips and notes about former tenants and rejected applicants, some kept since 2009. Our average reply to a data-protection access request fell from 31 days to 9.
Current focus
I am finishing the switch of our last 38,000 heat-cost allocators and heat meters to remote reading before the end of 2026, so that every centrally heated flat receives a monthly figure its tenants can understand. I am also extending our condition records to 1,100 buildings whose façades or heating plant are due for major work within five years.
Conversations I welcome
I am not looking to leave Haselwerder. With my management board's consent, I would consider one supervisory board seat at a German company with long-lived physical assets and many household customers, such as a municipal water or district-heating utility, alongside my cooperative seat. I can give about twenty-four days a year, and I accept a few speaking invitations.
Beyond the page
The record behind the perspective.
In the boardroom
Baugenossenschaft Lindenkamp eGMember of the supervisory board and its building committee · 2019–present
Formation
Diplom in StatisticsUniversität Haldenau · 1999
Certificate in Property Valuation, part-timeAkademie Ravensau für Immobilienbewertung · 2012
For years the comparison flats in our members' letters came from the same handful of buildings. Julian was the first person at the company to sit with us, show us the selection rule and change it when we found a flaw. We still disagree about rents. We no longer argue about whether the numbers are honest.
Renate OberkampFictional chair of the central tenants' advisory council, Haselwerder Wohnen SEThe full record ↗
Contact
I am not looking to leave Haselwerder. With my management board's consent, I would consider one supervisory board seat at a German company with long-lived physical assets and many household customers, such as a municipal water or district-heating utility, alongside my cooperative seat. I can give about twenty-four days a year, and I accept a few speaking invitations.