We looked into this fairly recently. If you're hiring trades one by one, you'll need someone to project manage if that isn't going to be you. If you're using a contractor though, they tend to self-manage. There are lots of contractors that specialise in residential renovation projects like loft conversions and extensions. They often manage most things themselves, including trades they need, permits, coordinating with building control, etc... or at least that's what we're finding now getting in quotes.

A friend who did an extension recently told me that there's still a lot to do as the property owner but you don't have to actually manage the build work. They also warned me though that you have zero clue about construction (like us! 😇 ), it'd be best to either get clued in quickly or look into engaging someone just to check in on standards. Many good contractors will self-regulate with foremen / owners ensuring work is done to a high standard but apparently standards often slip and if you don't know what's good and what's not you can end up with sub-standard workmanship. 

Ishan