Not legally, no. Building Regulations require professionally prepared technical drawings, but you're not required to use an architect specifically to get them. That said, a good architect earns their fee by making the space work better, not just look better: getting light, flow and proportion right, and steering you around planning problems before they cost you time or money. For smaller, simpler projects some homeowners go straight to a draughtsperson. For anything where the design itself matters, or the site has constraints, an architect usually pays for themselves.