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.

Permitted Development (PD) rights let you carry out certain kinds of building work, like many single-storey rear extensions or loft conversions, without submitting a full planning application, provided you stay within set limits on size, height and position. Planning permission is the formal application route, needed when a project falls outside those PD limits or where PD rights have been removed, such as in some conservation areas. Even PD projects usually benefit from a Lawful Development Certificate to confirm the work is compliant, since PD rules are easy to misjudge and getting it wrong can cause problems later, particularly when selling.

Planning permission is about whether your project is allowed at all: its size, position, and impact on neighbours and the area. Building Regulations are separate, and cover how it's actually built: structure, insulation, fire safety, ventilation. You can have planning permission and still fail Building Regs, or vice versa. Most projects need to satisfy both.

It depends on the size, position, your property type, and whether you're in a conservation area. Many single-storey rear extensions fall under Permitted Development and don't need a full application, but the rules have specific limits and exceptions. The safest first step is a proper assessment of your specific property rather than assuming either way.

Most projects follow four broad stages: Concept & Appraisal (working out what's possible), Pre-planning & Planning (developing the design and getting it through planning), Detail Design & Building Control (technical drawings and regulations submissions), and Construction (supporting the build). How much involvement you want at each stage is up to you.