You can move quickly if it's urgent, urgent. I think it's a balancing act, though. An extension is a huge chunk of cash. The costs are eye-watering. You want to make sure that you're sweating every quid of that for the best possible quality of space. There's no guarantee that you won't make mistakes even if you take your time, but they're more likely to happen in a rush.
A really good architect will present you with ideas you haven't even considered possible. For me, I'm not a particularly spatial person -
I find it difficult to orientate things in space, and I can never do those tests where you have to rotate an object in your head and say what it will look like. (I can, however, pack an awesome suitcase. Go figure!!). So I had to go through a whole mental process of sitting down and imagining the space in my head, including where everything would go, and where plug sockets would need to be etc. Eventually I had a mental picture of it, but along the way to it, I made decisions I didn't even realise existed at the start!!
I'm pretty naive and unpracticed at this stuff, though, and I reckon someone who was better at it could do it much faster! So a big chunk of this is me being crap!!