Do I need an architect or can my builder do it?

Do I need an architect?
Whether you need an architect for your project or can suffice with a builder, depends on your project and how much design input or creative thinking you require.
An architect does a lot more than just design some nice initial sketches to make your project look good. They will not only enhance your space with beautiful design features, but they will consider practical details and building methods, guide you through planning and even help manage the build. They will also have a commercial understanding of the project to make sure the design is feasible for your budget.
A builder may offer ‘Design & Build’ as a service. Builders do not tend to have the same training and design skills as an architect; they might not be able, for instance, to create something that considers the context of the building, or the way the light moves around the house during the day or how a family interacts with the space and each other. This approach is fine if you want something very simple that requires little to no design insight from someone other than yourself. This might suit you if you want to take a more hands on approach to a project.
If you hire an architect, you will always have someone that is looking after your interests and liaising with the contractor and consultants on your behalf.
Design & Build is often a good approach for loft conversions, for example, where there might not be much scope to design something creative and the main aim is to maximise the space.