Do I need to get plans drawn up before contacting a builder for quote?

We have recently purchased a three story town house (6 years old). There is an integral garage and two small rooms at the rear of this (a utility room and very small fourth bedroom). The kitchen is currently at the rear of the property on the first floor. Ideally we would like to move the plumbing for the washing machine into the garage and build an extension on the back of the house to have a kitchen/family room on the ground floor. We would like to knock the two small rooms into one and remove the external wall to create a large space as opposed to a conservatory/orangery type thing. Does this sound like a reasonable and doable project? And would we need to get plans drawn up before approaching a builder? We have an idea of cost in mind but could be completely off the mark. I'd hate to pay for plans to then get a quote to discover that the work would be completely out of our budget. Any advice would be greatly appreciated.