Incremental housing is a build model where the owner builds/buys only a part of the final home. Measures are taken to make additions easy, so that the house is affordable up front and can be expanded as needed. This is simply speaking - start with half a home at an affordable price, and add on as needed. OSE follows the incremental housing model with most of its builds, such as the Microhouse 1 and Seed Eco-Home.
See the concept explained:
6 Year Review of Open Source Architecture