Morning, Rocky.
A couple of things to bear in mind; you don't need an Architect (RIBA) to design anything, an Architectural Technologist/Technician is just as competent. Assuming you may mean 'someone who draws plans' rather than an actual Architect, there is no formula......we just charge based on time-taken and how complicated the project is.
There are 2-parts (usually) to a project.
1. Measuring up and producing 'before' and 'after' plans for Planning Permission.......and shepherding the application through the Planning Process.
(Although Planing Permission is not always required)
2. Producing annotated scaled drawings with all of the notes/information necessary for a builder to construct the project and to satisfy the Building Regulations.
I must admit, if *I* were pricing the job, an extension of that size wouldn't see me reaching £1 grand, let alone £1 grand more than the next quote!!!!
I really suggest that you look elsewhere.
If you want any more info, just ask.....:o)