What these builders are offering you it is not a “buying off-plan” business.
They want to avoid the regulation of a buying off-plan, so they offer you a different business.
They sell you the plot and you became “self-developer” (self promotor). This way, they avoid a lot of hustle and responsibilities that will be assumed by yourself.
Being a self promotor is quite a risky business. Never hired them as constructors AND project manager, as this will be like hiring the wolf to look after the sheep.