Introducing the Rentswatch API

Nicolas Kayser-Bril bio photo By Nicolas Kayser-Bril

Over the past few weeks, we built a simple API to make accessing Rentswatch data much easier.

The API is documented at While we encourage you to read the docs and try things out, let’s take an example and do one of these very popular rents-along-the-subway maps (see here for the map of Munich).

Making a map of rents across the subway

We’ll map the average rents along the subway line 3 in Paris.

Finding the stations list

Over at Wikipedia, they have a list of stations.

The list of stations on Wikipedia

This list of stations can be structured in a table easily with the IMPORTHTML() function of Google Spreadsheets. After a bit of cleaning, the list can be exported as a CSV.

The list of stations on Google Spreadsheets

Fetch data from the API in OpenRefine

Let’s import the list in OpenRefine, which we’ll use to ping the API and get the rents data.

The list of stations on OpenRefine

The Rentswatch API can return the average rents prices for any location in Europe. The location is geocoded using the Nominatim service of OpenStreetMaps (OSM), so that locations must be understood by OSM to be geocoded. Subway stations are present in OSM but require a certain format. In Paris, the capital letter M must be prefixed to the station name. First, let’s create a new column with the names to be geocoded.

The list of stations modified with capital M

Now is the important part. We’ll create the URLs that we will ping. We’ll use the “geocode” method of the API. It requires an address and a radius and gives the average rent price for all flats within this radius. Obviously, you’ll have to replace YOUR_API_TOKEN by, well, your API token.

The list of stations with URLs

OpenRefine can now go and ping all these URLs to find the average price along the line. The API returns a bunch of JSON data.

JSON files for each station

Let’s parse this data to get the average price for each station.

Parse data for each station

Make it legible

Let’s multiply the average price per square meter by 40 to have the average price for a 40-square-meter flat, which is more ligible for users.

New column for a 40 sqm flat

Now open an image processing software and have fun!

Mock metro plan for line 3

Want an API key?

Rentswatch aims at fostering quality journalism on the housing crisis. We are looking for exclusive media partners in Europe to explore the data with us. Contact us at