Hi there, Architect here - perhaps I can provide some extra information!
The process of construction and working with an architect is as follows:
Survey
Concept Design
Detailed Design
Technical Design
Construction
Hand-over
That all sounds like quite alot, but its all designed to ensure that you, the client go through a series of steps that put you in the best position to have a successful build process.
I'm going to provide a bit of further information on the stages I listed above:
Survey - a detailed measured survey of your property is designed to provide you and the construction team with an accurate drawn representation of your home. This should include lots, but the key information is: measurements of each room pertinent to the works, all changes of level, boiler, gas/elec boxes, manholes, window and door info, downstands (usually denotes drainage or structure), garden dims (if relevant), neighbouring information (size of existing extensions etc), ceiling heights.
Concept Design - this is fairly self explanatory! Its designed to offer you the opportunity to work with your architect to generate the initial response to the brief.
Detailed Design - Developing on the concept design, the architect will now help you take the design forward to planning (if necessary). This will include finalising things such as layout, design, materials, and working with any relevant pre-planning consultants (often relevant on tricky sites, or where a basement is involved).
Technical Design - This covers building control and construction information. Here, your architect will ensure that the design meets all relevant building control documents, and that the contractor has all the information they require to build your extension. This includes (but is not limited to): drainage information, finishes schedules (doors, finishes, flooring etc), kitchen design and specification (working with kitchen companies if necessary), construction details, wall floor and roof build ups, thermal efficiency of the build etc. They will also work with a structural engineer to ensure the structural integrity of the build.
Construction - If you choose to have your architect administer the building contract between you and the contractor, then they will visit site weekly to check on the progress of the works, and make sure you are getting what you are paying for. They will also handle the valuations of the works to make sure the contractor is paid what he is owed for works undertaken.
Hand-over - finally, you get to move in - once of course the contractor has completed all works, and handed the property back into your possession. There will of course be snagging (minor corrections done whilst the construction team are still on site), and then defects 6 months later (often related to cracks that appear as a building settles etc).
There is of course LOTS more involved in the process, but get yourself a competent Architect who can guide you through the process - the cost of employing them will be more than worth the peace of mind, control and attention to design and quality you receive!
Best of luck, and feel free to give me a shout if you (or anyone else reading this) needs any more help or advice.