When you decide to have a house extension, loft conversion or home modification, there are a lot of choices to be made and the route you decide to take will hugely influence the outcome.
It seems cheaper and easier to not involve anyone other than the builder at first, allowing them to be at the helm of the project. Ultimately you are placing all your trust in them and them alone.
What if, as a result of crossed wires between you and your builder, you thought a critical aspect of your design was included in the builder's quote and, half-way through construction, it is revealed that it was not included? You are left in a situation where you can not afford the extra cost, or there are so many add-ons occurring due to this miscommunication that your initial ideas become unaffordable.
What if the builder says an aspect of your idea cannot be built and as a result ruins the design you were after, when in reality it could have been built but maybe not by that builder? You would never know. A professional there with specific knowledge and expertise would be extremely beneficial.
By using independent experts with specialist skills (who have no bias towards the builder) who have your project's interests at heart, and are able to prepare designs and drawings will vastly help to avoid such troubles. It will also help the project go according to plan and may even make your idea become better than you imagined.
In most instances, you will be doing the builder a favour by using such professionals.
Many builders like straight forward jobs and preparing the project in advance of their involvement takes the sting out of the tail.