i build my own stuff around my raspberry pi. i've got motion detection, temperature sensors, lights, garage doors, plant irrigators. it's all fun, but be prepared to spend a lot of time. If you are making this yourself, you are taking up electrical engineering, mechanical engineering, software engineering, carpentry, plumbing, electrical work, running to home depot. Everytime i do a project i tell myself, next time i'm just buying something.
then i come back and do another.
just be realistic about what you want. do you want good home automation? or do you want to putz about with computers and components? (for me, it seems to be the later)