Jesting aside, architects and ATs are a bit like doctors and nurses.. Doctors get all the credit but it's the nurses who, on a day to day basis, know: what they are doing, how to treat the vast majority of cases better than doctors and how to actually do it
If it makes you feel any better, I drw the plans for my current semi pair project myself. Drawing anything is the easy part. Drawing something that can actually be built and has a hope of conforming to regs is the hard part. I lost countless hours of me life reading the approved documents learning the less obvious stuff about why I couldn't put doors and rooms willy nilly throughout the place. I didn't get any building control plans drawn, I just discuss it with my building control officer (this is called the building notice route, though independent inspectors don't refer to it as that. If you're confident your builder knows the regs, then he can chat with the officer, build it and then get it inspected. someone Bears the cost of rectifying anything the inspector is unhappy with). If you engage an AT he will draw plans that comply with regs and for a minor extra fee can annotate and detail anything regs wise, so your builder can just crack on. He may also do minor structural stuff himself like window and beam calcs. If your extension involves a crazy amount of architectural extravagance, he will know an engineer who he works well wi and is good value.
And finally, for a bit of perspective, my scheme was 1100 for building control, 2650 for structural engineers fees, 795 for planning application and the drawings took me about 6 months because of constant minor changes, dialog with the planning dept and learning. In practical terms of time value of money, that's probably about 15,000 quids worth of late nights ignoring the wife and son. hiring an AT would have cost me far less, but at least I can say I did it.. That and the 3D model I drew has been massively useful in generating 2d plans at any slice anyone wants, as well as virtually walking round and checking it all makes sense
Never confuse cost wih value
and never underestimate the value in having well drawn up, exacting plans. They form the core of your relationship with your builder in terms of his estimating/quoting and you getting what you want. If you aren't confident your builder knows he regs, get your AT to draw or state all the necessary bits so you know what to ask your builder for and have a happy inspector