The best moment you ask the opinion of your builders, and of anyone else for that matter is before starting the build itself. That way you will have time to consider the opinions you receive and decide if you want to use them or not. It is true that you can ask for, and get, suggestions and ideas during the build to. But integrating them can cost you a lot of money. Not to mention that modifying the initial project will take time and a lot of adjustments from both you and your builders’ team.