Who should I contact first for my renovation project

An architect? A builder? A structural engineer… or an interior designer?

Firstly, there is no legal requirement to engage an architect when designing/building an extension  (which is actually quite bizarre)

If you are extending your home, i would strongly advise engaging an architect, and the main reason for this (alongside hopefully great design) is to ensure your building is safe and legal

That said, architects and interior designers vary wildly in their skills and experience. 

Some architects have little experience in lighting plans, electrical plans, interior hard finishes (engineered wood / tiles / paint specs,) whereas some architects are a complete hybrid and can offer the full service. 

Some interior designers focus more on the soft furnishings and window treatments, whereas others can draw up detailed drawings packages, lighting plans, electrical plans, bespoke joinery, the works.

In my personal experience, many friends have hired architects ‘up to planning’ and then the builder has taken things from there, this means the only information your builder has to work with is an empty shell of a building.

Exmaples:

• You would like to knock down an internal wall and reconfigure your space

I would firstly consult a interior designer (with architectural ability) and a structural engineer at the same time 

• You would like to extend your home

I would consult an architect and interior designer at the same time, or an architect with the capability of delivering the full ID package