You've got an architect. Do you need an interior designer too?
In most renovations, it helps to have both. The roles are complementary: the architect shapes the building and how you'll live in it, and the interior designer shapes what fills it. The best results come when the two work together from the start.
1. Each role has its own focus. A good architect is focused on how people will live in a space as much as on the technical design and innovation of the building. A good interior designer is focused on how you'll live in it too, through the furnishings: the furniture, lighting, soft furnishings and art. Hard surfaces such as flooring, tiling and stone can be specified by either, while the soft furnishings, lighting and art are best specified by the interior designer, as part of one scheme for the whole home.
2. Early alignment makes the scheme more coherent. When statement pieces, colour schemes and key materials are agreed early, the architecture and the interior can be designed around each other. A fireplace wall can be sized for the painting that will hang above it, and a window can frame the view from the chair that will sit beside it. The result is a more coherent and thoughtful home.
3. Lighting has to be decided before the build begins. Many of today's LED fittings need their own drivers, and some need particular wiring or dimming controls. All of this has to be specified before construction starts, while cables can still go exactly where they're needed. Lighting is one of the clearest cases where early decisions by the interior designer save disruption and cost later.
4. Good architects and interior designers work collaboratively. The best projects bring the architect, the interior designer and the client together, each contributing what they know best. With more minds working well together, the home that results is better than any one of them would have designed alone.
5. Where an architect's strengths are more technical, an interior designer matters even more. Some architects are technically proficient but bring less of a client-centred view. In that case, it's critical to have an interior designer who understands the client's tastes and how they live, and who makes sure both are represented in the architect's designs.
It's why, wherever I can, I join a project while the drawings can still change, and work alongside the architect from there. For more on how the interior of an extension can carry the design language of the original house, see What is proportion in interior design?