Is it legally binding for an architect to demand an interim payment?

Firstly what do you mean by legally binding? Any seller is within their right to ask for a deposit payment, for services or goods they are to supply. It is up to the buyer of the offered service to decline or accept this request. Both buyer and seller should agree terms and enter into some form of contract, to legally cover their transaction. All Bona Fides suppliers should provide a " Terms of Business" agreement for their customers. This would cover all payment matters for the goods/services supplied. Regards, Allan Broadway Construction.