Current Stonebridge Park depot was built as stage 1 on an odd shaped site, on understanding that adjacent land would become vacant for further roads. It was needed in a hurry to vacate Neasden for the Jubilee fleet. I expect it will maintain the coming larger fleet of new trains, with their reduced maintenance needs. What is needed is a reasonable stabling area to serve the south end of the line.
Far better to forget all this and revert to extending to Camberwell Green as origionally planned . C G is an obvious traffic center with not very good N R access within short walking distance. Then possibly extend to Dulwich area. I'm sure that Hayes residents would be delighted to swap their present trains for yet another overlong tube journey as with Epping etc: