Before you go ahead with your plan to build a garden room, you should check whether your neighbours will give you permission to add a new structure to their property. You may need to consider the provisions of the Party Wall Act. Once the neighbours’ permission is received, you can then consider how to proceed. If you can’t get your neighbours permission, build it away from the boundary. Note that the provisions of the party wall act still apply if you’re within 3 metres of the boundary.
A garden room can be built under Permitted Development rights, subject to limits on height and space. Where PD rights are not available, you will need to apply for planning permission.