Garage with utility room, worth insulating utility to make gym?

[sbplanet]

I'm wondering what's the best way to make most use of our utility room. It is the end part of a single storey, flat-roofed side extension, at the rear of the garage. It's entry door is external, there's no direct entry to garage or utility from the house.
The walls are currently bare concrete block, and the flat roof bare joists with no internal ceiling. There's a wall with a door separating the garage from the utility, and the only light source is two windows in the utility room.
I'm getting the utility room old pine-framed single windows and external wooden door replaced. I think I will also replace the door from utility to garage, does it have to be a fire door? I was thinking of putting a half-glazed upvc door to let light into the garage?
Any idea whether to bother insulating the utility room as although currently it's just used for laundry we might put some gym equipment in. But I didn't think it would be worth the extra (large?) cost in insulating the room. Would probably just paint the walls and concrete floor.
Thanks.

In one house - I'm pretty sure that the door between the garage and utility is not a fire door.

In the other - well it depends which you need most, the garage or a gym. This garage doesn't have a door into the house. Personally I have one car nowadays and prefer to keep it in the garage, because there are trees outside my house and if I park outside the birds poop on the car. But my neighbours 2 doors down don't have the trees but did convert the garage - and leave their four cars outside!

Are you planning to move eventually? I'd speak to an estate agent and see how much the gym/potential office would add to value.

Would still have the garage (one car size anyway), although used for junk/garden stuff.
Not planning on moving soon, we've only been here 16 years! :D
Didn't know whether if we didn't insulate the room wouldn't get used - I mean using home gym equipment isn't top of our list of stuff to do even in lockdown! Or will it mostly be warm enough with double-glazed windows, doors and a ceiling?