There are some building firms which specialise in extensions, who will probably have in house architects and structural engineers, but if you want a decent range of quotes, having your own drawings means you can approach a wider range of builders for quotes.
We did a side extension, got an architect and the drawings, then did the 3 quotes thing, but it was painful just finding 3 different people to come round and cost it up. We had people not turn up (even though they were recommended), someone who asked me for information about what sort of bricks I wanted to use (I didn't know, is there not a standard brick??) before he'd even consider giving a price and it took nearly 6 months just to get an approximate price from 3 different people. Quotes were from 30k - 70k, we chose the £40k which seemed realistic, but we overspent by approx £10k so don't believe the quote as the actual cost.
Financed with savings and a remortgage, we had to have a valuation for the remortgage and we were fortunate to have a fair bit of wriggle room on the valuation, as it was low and the valuer did a lot of umming and ahhing over bits we were going to knock down or redo.
Work was quoted as 10-12 weeks, took 9 months.
Then we had DC3 and went on to have a loft extension. Got 3 quotes within a week of asking, although 9-18month wait to start work. All quotes within £1000 of each other and included drawings, certificate of lawfulness, etc.
Work was quoted as 8-10 weeks ,took 4 months. Extras were only about £1k and nothing we hadn't been made aware of (bigger skylights, spotlights and extra plug sockets).
Loft extension was definitely easier process for us, but either way prepare for it to take more time and money than you imagined!