You can build your own car in the UK. If you build from a kit then the kit will typically have type approval and you just pay for your build to be inspected. If

You can build your own car in the UK. If you build from a kit then the kit will typically have type approval and you just pay for your build to be inspected. If you create something unique it needs a more expensive inspection that incorporates type approval. Open source would be like a kit where you make the parts. I think legally this would be treated as a custom build. The full inspection is £450, which seems quite reasonable.