@Meandmrsjonesgotathinggoingon have a look at modular extensions, they're about a third cheaper.
We're having one done - work starts this week. Bricks are really expensive and also slow to build, so you have a variable and large labour charge. Modular extensions are built from metal and wood, and they usually go up in about 2-3 weeks.
We're late adopters in this country. In Europe and the US, modular homes are really common. They're starting to be built over here but still have a long way to go.
Modular builds are REALLY well-insulated, and they look just like a regular build inside. On the outside you can pick between render, brick slip and other finishes (can't remember). We're putting our kitchen and dining room in our extension, and we're building an annexe for DM.
The only real negative I would say is that I don't think they're universally accepted by mortgage companies as they're a non-standard build. They're not the same as the old concrete non-standard builds of the past. Modular builds are built to last and exceed the standards of a traditional brick build, hence why lots of mortgage firms are now willing to consider them.
For us, it was a no-brainer. The cost dropped from about £150k to £100k, very roughly. The cost is fixed at the start, so once you have a price it doesn't increase - no contingencies needed.
Not all companies are the same though so make sure you use one that's thorough and properly regulated. There are some out there who are cheaper but I had bad vibes about their professionalism.
Just a thought in case it helps.