Is it legal for a developer to require 10% deposit on real estate, which is completed new construction, not under construction?

If the home is in a new development, the developer can sell it with or without an initial deposit amount. If you do not want to put down a 10% deposit, you are free to say "No" and find another development. However, it would be unusual to find a new developer that would not require a deposit amount.

There is no Florida law that sets the amount of a deposit in a real estate transaction. The amount of the deposit, like the purchase price is something to be negotiated between the parties. If you cannot agree on contract terms, find another property to buy.

There is no law preventing the developer from demanding 10%, but developers are struggling in this market, and you have the upper hand . Make sure that you counter-offer, or just walk if need be. You may want an attorney or realtor to negotiate on your behalf.