As a sole trader who wants to use the HMRC Simplified Expenses, where you can claim 45p per mile for business journeys, can I claim the return journey back to my house? In other words, can I only claim the journey to the client or the round trip that includes the journey back to my house?
You claim for the mileage that you drive wholly and exclusively for business purposes - if that’s an out and back journey to a client then you’d claim both ways, if you made a triangular journey from home to client A to client B and back home then you’d claim the three legs of the triangle, etc.
The exception is journeys between home and a “usual place of work” - if you’re always travelling to and from the same location (on a long term contract for the same client) then that could be considered commuting rather than business travel, I’d check with my accountant in that case.