Design and workmanship are the true tests of a good build
however, he does have the power to decide if the quality of work is of the correct standard and ask for elements to be redone should if necessary.
This is also confirmed in the Royal Institute of British Architects handbook on the Appointment of an Architect, where it states that the role of the architect is to see that the work is “generally in accordance with the contract documents”. The important word to note here is “generally”. Even if your architect was to visit the site daily it would still be impossible for him to guarantee that absolutely everything is built correctly. For the kind of supervision to take place that meets with your expectations a constant site presence is required. Larger projects often have a clerk of works acting as the eyes and ears for the architect whilst monitoring quality.
Domestic extensions do not generally justify this expense consequently; a lot more responsibility rests with the builder. Hopefully your architect carried out the correct research and took up the necessary references or had previous experience of working with them before making the appointment. This should give you a degree of confidence in their ability to do a good job.
It is common knowledge within the profession that on traditional jobs, the construction phase of work can swallow up enormous amounts of time and therefore fee, often making them very unprofitable. However, this is no reason for your architect not to fulfill his obligations. Depending on the size of project and the length of the construction period it would be reasonable for him to make a brief visit either weekly or even fortnightly. It may be an idea to suggest that he carries out some unannounced visits although I am confident there will be a lot of dialogue going on between him and the contractor you are unaware of. In addition, it is generally better to inspect elements of work before they are completed and covered up from view, this may explain why his visits do not appear to relate to specific stages of work.
You do not say anywhere that you are dissatisfied with the quality of the design or workmanship; surely this must be the true test?