Most of our houses have parking for two cars outside them. What will we do with these existing spaces then?
1. Rent the spaces out to autonomous cars. Some will do this but their ability to be used more of the time and to park themselves densely in unpopulated areas means we might have better use for the space.
2. More garden.
3. New houses. My two car spaces take up 25 square meters. Which is twice the size of this tiny house.
Or 25m squared is half the floor space of my actual house. And of this Ikea house.
These houses are cheap and I doubt people will be too bothered by having one replace the parking spaces behind their house.
People having a small house at the end of their garden might be already happening. For example this article Why An Increasing Number Of Americans Want To Build A Granny Flat. Explains why more people are already building houses beside their current one. Both young adult children and aging parents might find these small houses preferable to the alternatives. With young people increasingly living at home at an older age and seeming to have higher debts for worse job prospects a granny flat becomes more attractive.
It is possible autonomous cars will end up meaning people live in bigger houses further out of the city. But as a way to retrofit current housing car space houses will be popular.
But if all these car parking spaces become free. And you share with your neighbour enough space build a house the same size as yours. There will be some people who try and build new housing there.