Restricting height to 2m is a bit harsh, a tall person wouldn't fit inside by the time you take into account the roof structure
Have you thought about having a go yourself? It's not that hard to lay bricks once you get into it, the mortar mix is important to get right. If you have a wall you can't see start there, as long as it's structural you can get away with a bit of a sloppy couple of courses. I built the bunglalow below with the help of a mate, no trades used except a plasterer. I'm a project engineer so I know what the plan is but had little hands on experience. All the materials came to £35k, but that includes £8k for services/boiler/treatment plant so £27k all in.