Custom BAG API integration development
Appfront builds custom integrations with the Kadaster's BAG API, so your application always works with current and authoritative address and building data. Using the BAG API Individual Queries and the PDOK Location Server, we validate addresses, retrieve residential units, buildings and address designations, and keep your own records in sync with the source. Fewer typos, less manual work, and a process that demonstrably relies on the right data.
What is a BAG API integration?
The Key Register of Addresses and Buildings (BAG) is one of the Dutch basic registers and is maintained by the Kadaster. The BAG contains all official addresses and buildings in the Netherlands: residential units, buildings, address designations, public spaces and localities, each with a unique identifier and geometry. It is the authoritative source that government and business use to work with reliable address data.
A custom BAG integration in practice means: validating addresses by postcode and house number in forms and portals, retrieving residential units and buildings with usage purpose, floor area and year of construction, looking up public spaces and localities, and keeping your own data in sync with the current BAG. Alongside the Kadaster's BAG API Individual Queries, we use the PDOK Location Server where appropriate for fast geocoding and address autocomplete.
Appfront builds according to the official BAG API documentation and the OWASP ASVS security standard. We tune queries, caching and error handling to your actual processes, so the integration grows with your application and address lookups remain reliable and fast even at peak volumes. For related government integrations we also build Haal Centraal, BRP and ZGW API integrations.
Authoritative address data
Residential units, buildings, address designations, public spaces and localities directly from the Kadaster's source, not outdated address lists. Always current, with a unique identifier and geometry for each object.
Address validation in forms
Visitors enter their postcode and house number and see the street and locality filled in automatically. That prevents typing errors, speeds up completion and ensures only existing addresses enter your system.
Kadaster & PDOK combined
The full BAG objects via the Kadaster's API, supplemented by the open PDOK Location Server for fast geocoding and map layers. For each use case we choose the source that fits best: completeness or speed.
Our development process for BAG API integrations
We work to a proven methodology that removes uncertainty early and delivers a stable integration. From an initial analysis of your processes and the BAG objects you need, through to go-live and ongoing management, every step is aimed at an integration your team can understand and trust.
We map out which BAG objects you need (address validation, residential units, buildings, geometry), whether the source is the Kadaster or PDOK, and how the data flows into your application.
We design the integration architecture, arrange the Kadaster API key and caching, and define an error-handling and refresh strategy for BAG mutations.
We implement the queries with automated tests, structured logging and monitoring. You see working builds along the way.
Controlled go-live with data validation and a safety net, followed by ongoing management, monitoring and further development.
What a BAG API integration delivers in practice
Every BAG integration is tailored to your type of process, data needs and surrounding systems. Below are the functionalities we most often deliver for organisations working with address and building data.
Address validation & autocomplete
Enter a postcode and house number, and the street and town are filled in automatically via the PDOK Locatieserver or the BAG API. Only existing addresses enter your system, with fewer typing errors and faster input for your users.
Residential unit & building data
Retrieve complete object data: residential and other usage units with purpose and floor area, and buildings with construction year and status, linked to the address number designation. Ideal for property, valuation and permit processes that need more than just an address.
Geometry & geocoding
Retrieve coordinates and outlines of buildings and residential units for display on a map, spatial analysis or determining which objects fall within an area. Through PDOK WFS/WMS we combine BAG with other geo layers.
Mutations & updates
The BAG changes continuously through new construction, demolition and amendments. We set up how your application stays current, either by querying in real time or by refreshing periodically, so you never rely on outdated address or building data.
Integration with your line-of-business application
BAG data flows directly into your case management system, CRM, property or permit application. Address and object data are transferred consistently, so staff no longer retype information manually and your records match the source.
Caching & performance
Smart caching of frequently requested addresses and objects keeps your application fast and spares the source API. With monitoring and alerting on response times and error rates, address functionality remains reliable even at peak volumes.
Typical use cases in practice
A BAG integration looks very different from one organisation to the next. We regularly see a number of recurring contexts, and for each of them we have a working setup that takes into account the right BAG objects, source choice (Kadaster or PDOK) and alignment with your line-of-business application.
Municipalities & VTH
Municipalities and environmental services use BAG data in permit issuing, supervision and enforcement (VTH). Addresses and objects are recorded unambiguously and linked to cases and locations. Also read about software for municipalities and the ZGW API integration.
Housing associations & property management
Housing associations and property managers link their portfolios to residential units and buildings from the BAG, including purpose, floor area and construction year. This keeps the property register up to date and demonstrably based on the source. See our housing association software.
Estate agents & valuation
Estate agents and valuers automatically enrich properties with BAG data: construction year, floor area, usage purpose and building status. Address validation and object linking prevent errors in files and reports, and speed up building a complete picture of each property.
Energy label & WOZ processes
For energy label, WOZ and sustainability processes, the residential unit is the unambiguous key. By linking BAG objects and geometry, data from different sources align, and assessments remain traceable to the correct object.
Technology we use
We build BAG integrations using the Kadaster's official BAG API Individual Queries v2 and, where appropriate, the open PDOK services, combined with the backend stack that suits you. The precise choice depends on your use case and source preference, so your own team can manage or further develop the integration.
For the base map with roads, water and terrain in your own systems, see our page on a BGT integration.
For registrations whose information model is based on MIM, see our page on a MIM integration.
For municipalities that, as data owners, want to track mutations from signal to delivery, there is our software for BAG mutation management.
Why choose Appfront for your BAG API integration?
Appfront has extensive experience building integrations with base registers and government APIs for a wide range of organisations in the Netherlands. We always start with a thorough analysis of existing systems and processes, because an integration must not only work technically but also add practical value to how you work.
For every integration, we write clear documentation and make sure your own team, or any future supplier, can understand and manage it. No black box, just transparent code and clear agreements on monitoring, alerting and maintenance.
You work with a dedicated point of contact who understands both the technical and the functional side. This keeps communication short, prevents misunderstandings and speeds up decisions when choices need to be made during development.
See also our services on municipality websites, custom software and middleware.
- Experience with the BAG API Individual Queries and PDOK services
- Experienced with base registers and government integrations (BAG, BRP, Haal Centraal, ZGW)
- Experienced in address validation, geometry, geocoding and BAG mutations
- Secure by default: API key kept in a secure vault, TLS, scoped and least-privilege permissions
- Structured error handling and retry mechanisms
- Comprehensive logging and monitoring from day one
- Clear documentation your team can read and manage
- A fixed point of contact, no account managers passed around
- Ongoing maintenance and proactive further development
- A way of working aligned with your existing IT landscape
Security and privacy in BAG API integrations
The BAG does not contain special category personal data, but address data and the way you process it call for care. Appfront builds to the OWASP ASVS security standard. This means, among other things: the Kadaster API key is kept in a secure vault, traffic is encrypted over TLS, access follows least-privilege principles, and queries and data flows are audited regularly.
We only process the address and building data your process requires, apply data minimisation, and document the data flows so your record of processing activities is complete and you can demonstrate compliance with the GDPR. Structured logging makes every query traceable, so you stay in control of what happens to the data.More on our security approach: information security policy and CVD policy.
- GDPR-compliant data processing and data minimisation
- Encryption in transit (TLS 1.2+) and at rest
- Role-based access and least-privilege principles
- Audit logs with traceable data flows
- Automatic retries and robust error handling
- Monitoring and alerting for anomalies
- Kadaster API key in secrets management
- Documentation for your record of processing activities
Frequently asked questions about BAG API integrations
Answers to the questions we are asked most often about integrations with the Basic Registry of Addresses and Buildings.
A BAG API integration is a technical link between your application and the Basic Registration of Addresses and Buildings (BAG) of the Kadaster. Via the BAG API Individual Queries, you retrieve real-time address and building data (residential units, buildings, address designations, public spaces and towns) in JSON. An integration can be simple (address validation in a form based on postcode and house number) or extensive (full property data, geometry and history in a specialist application or dashboard).
The BAG API Individual Queries from the Kadaster delivers the complete, authoritative BAG objects (residential unit, building, address designation, public space, locality) including geometry and history, and requires a free API key. PDOK publishes the same source data as open data via, among others, the Locatieserver (geocoding, suggest, lookup, reverse) and WFS/WMS services, which is ideal for fast address autocomplete and map layers without authentication. Which source is best suited depends on your use case; we will advise you on that.
The BAG contains all addresses and buildings in the Netherlands: residential objects with usage purpose and floor area, buildings with construction year and status, address designations (house number, addition, postcode), public spaces (street names) and localities. Each object has a unique identifier and geometry. Through the API you link this data together, validate addresses entered, and keep your own records in sync with the authoritative source.
We work with the official BAG API Individual Queries v2 (RESTful JSON, HAL links) from the Kadaster and, where appropriate, the PDOK Location Server and OGC services (WFS/WMS). Authentication runs through the Kadaster API key. We build the integration in the backend stack that suits you, whether Node.js, Python, PHP, .NET, Java or Go, with caching, error handling and monitoring so that address queries remain reliable even at peak volume.
The costs depend on the complexity of the integration: address validation on postcode and house number alone is lighter than fully exposing property data, geometry and history in a specialist application. The number of systems to integrate, caching strategy, and ongoing management and monitoring also play a part. The BAG data itself is open and free to query. We always provide a clear quote after a no-obligation analysis of your situation.
Yes. The BAG contains no special category personal data, but address data calls for careful handling. Appfront builds to OWASP ASVS: the Kadaster API key is stored in a secure vault, traffic is encrypted (TLS), access follows least privilege, and logging is structured. We process only the data your process requires, document the data flows for your record of processing activities, and ensure that your application demonstrably complies with the GDPR.
Yes. Appfront regularly takes over existing integrations, including those set up by another party. We review the API integration, caching, error handling and the way address and object data are maintained, document the current setup, and propose improvements. From that point on, we handle adjustments, extensions and monitoring, including timely refreshing of BAG mutations.
A BAG integration is valuable for any organisation working with addresses and buildings: municipalities and environmental services (permit licensing, VTH), housing associations and property managers, estate agents and valuers, energy label and WOZ (property valuation) processes, and businesses that need address validation in forms and portals. Wherever reliable, up-to-date address data makes the difference, the BAG provides an authoritative source.
Ready to have your BAG API integration built?
Tell us which address or building data you need and in which application the BAG should land. We are happy to help you think through address validation, object data, geometry and the choice between the Kadaster API and PDOK. Feel free to get in touch; a no-obligation first conversation will quickly give you a clear picture of the possibilities.