What Mietfix does
For private landlords with one to about five residential units.
- 01
An invoice behind every line
Every line names the invoice, the legal provision and the calculation.
- 02
Change of tenant to the day
Fixed costs by days, consumption costs by the interim meter reading.
- 03
Vacancy
The landlord bears the costs of an empty flat, not the other tenants.
- 04
Heating Costs Ordinance
Consumption share of 50 to 70 per cent. Separates heating and hot water from one system. Estimates when a meter fails.
- 05
Twelve-month deadline
Mietfix shows for each tenancy the date by which the statement must reach the tenant.
- 06
Reading invoices
A language model on the user's own computer suggests the items on an invoice. The landlord checks and confirms them.
- 07
Profitability
Investment appraisal, repayment schedule and household budget for the property.
- 08
Open files
JSON and PDF files, readable even without Mietfix and without a licence.
Example: change of tenant
A tenant who moves out on 15 May has lived there for 37.0% of the year. But he has used 54.9% of the heating demand, because the winter falls in his period. That is why the meter reading on the changeover day counts. If it is missing, Mietfix splits the costs under § 9b(3) HeizkostenV and says so.
Reading invoices with a language model
Every suggested item is marked „vom Sprachmodell“ (from the language model). It only counts once a person has confirmed it. Without a language model, only the automatic split into items is lost.
No legal advice
Mietfix calculates and documents. It does not assess individual cases.
The landlord decides
Whether an item is recoverable and what the tenancy agreement says is for the landlord to decide.
No guarantee
Mietfix gives no guarantee that a statement under the BetrKV or HeizkostenV is correct.
No tax or investment advice
The tax in the investment calculator is greatly simplified. It is no substitute for tax advice.
Degree-day figures not official
The standard with the official degree-day figures is not yet available. The values used are therefore not officially verified.
Requirements
A Windows computer is enough.
System requirements
- Windows 10 from version 1809, or Windows 11, 64-bit.
- WebView2. It is included in Windows 11. If it is missing, the installer gives the link.
- 73 MB disk space.
- No administrator rights needed.
- Optional: Ollama, for reading invoices with a language model.
What Mietfix cannot do
No owners' associations
No owners' association charges (Hausgeld), no VAT.
Windows only
No macOS, no Linux, no phones.
Privacy
Tenant data stays on the landlord's computer. Timo Britz receives none of it.
- Update check: At most once a week Mietfix asks timobritz.de whether there is a new version. No data from the program is sent, only the IP address.
- Can be switched off: in the program under Legal („Rechtliches“). After that, Mietfix no longer connects to anything on its own.
- Only on click: Mietfix opens the product page in the browser or downloads a language model through Ollama.
- Not encrypted: All data, including the names of tenants, is stored as readable files in
%APPDATA%\Mietfix. Anyone who shares the computer needs a protected user account or an encrypted drive. - The controller under the GDPR is the landlord. Timo Britz has no access to the data. A data processing agreement is therefore not needed.
Full details are given in the privacy policy.
Legal: provider, connections and stored data are also shown in the program.
Price and licence
One licence covers all 1.x versions. No subscription, no activation, no account.
Free to try before buying.
- All 1.x versions, no expiry date
- Security updates until at least 31 December 2028
- No activation, no account
- Licence check without internet
No legal advice. No guarantee that a statement under the BetrKV or HeizkostenV is correct.
Note: the program's interface is available in German only. Only the German version of the terms of use is legally binding.
Viewing, preparing statements, printing and exporting are never blocked.
Free trial
Without a licence, Mietfix manages one house with one billing period.
Licence
A signed text that is pasted into the program under Licence („Lizenz“). Licences for other programs by Timo Britz do not work for Mietfix.
Download
One installer for the trial and the full version.
Mietfix 1.1.1
Ollama and the language model are not included.
SHA-256: edfe50630bf7d3276d3982cc2bac46c5f26c96f4edd16427af42f48ae1be4351
Storage locations
- Program
%LOCALAPPDATA%\Programs\Mietfix - Data
%APPDATA%\Mietfix
Uninstalling keeps the data. Mietfix itself deletes no data.
Not code-signed. Windows SmartScreen therefore warns on first launch. Continue with „More info“ and „Run anyway“.
On a newly set-up Windows 11, Smart App Control may block the program entirely.
Versions
- 1.1.1
-
29 September 2026
New program icon and revised texts. The calculations stay the same.
Earlier versions
- 1.1.0: Kostenlage is now called Mietfix. Existing data is taken over on first start.
- 1.0.1: bug fixes, among them in the update check and the program window.
- 1.0.0: licence and installer. First complete version.
- 0.3.0: reading invoices with a language model, profitability tools.
- 0.2.0: the user interface.
- 0.1.1: legal provisions checked against the official texts.
- 0.1.0: first version: calculating and documenting.
Questions
Questions about the program or the licence can be sent using this form. Legal questions about individual statements are not answered.
- Provider
Timo Britz, Cologne. Address, telephone and mandatory details are in the legal notice.
- Price
€29 one-off including VAT, for all 1.x versions.
- Withdrawal
Digistore24's cancellation policy applies. No waiver of the right of withdrawal is required. Consumers can withdraw within the statutory period of 14 days, even after receiving the licence.
- Terms
Usage rights and liability are set out in the terms of use.