Will Arnold is Head of Sustainable Materials at the Useful Simple Trust, Technical Author for the UK Net Zero Carbon Buildings Standard, Convenor of Part Z, and Visiting Professor at the University of Bath. Photograph by Steve Cross, Instagram @stevecrossphotos. ‘Do you lobby here often?’ the housebuilder asked me, grinning. It was my first time in the Houses of Parliament. We were both there for an event on embodied carbon, and I was hopeful that Part Z might finally gain some political traction.