Didn't go into detail yet, as I'm not sure. Last year was my 50th, and for a while I was considering rewarding myself by splashing on a decent watch (there might even have been a thread about it). But I never pulled the trigger, so sat on that bit extra
We've got quite a reasonable back garden - small by US standards, but with an old style outbuilding. We're pulling that down to put a new brick built double garage in, and had planned a micro granny flat. 2020 made me decide to put the watch fund towards gym equipment - Plans change 
I want to run water and electric to it, so we've got someone to draw up a proper planning application - not sure how long that will take
I know you didn't ask for that detail, but it contextualises what I'm hoping to do - pretty standard stuff, boring almost
Mirrored wall (of course), fixed dumbell rack, Smith Machine (sorry but I like them more than racks), couple of adjustable benches, Olympic bar and rubberised plates, e-z curl bar - some sort of chin and dip stations. Was thinking of maybe some sort of hack squat or leg press but hadn't really researched options
I think it was you that recommended a nicely padded floor, so I'll be looking into that. My trusty old Air-Dyne will be moving in, and my Wife really wants some sort of punchbag or strike dummy - guess she's got 'things' 
Must be missing some obvious stuff here, but it's unlikely to be up before Autumn, so I've got plenty of time to look around
Guess I'll brag first, like usual. Awfully excited about the mats so yeah I likely mentioned it. I scored German made Zebras, 40mm, which are really well regarded, half price as ex-tournament mats. There not wildly expensive new but, due to covid restrictions, they didn't see much action and are in mint nick. My understanding is that Fuji are also top level, and the Zebra dude (whose name eludes) went across and took the process with him. I don't know much about Swain mats but they enjoy a good rep as well, despite being Chinese manufactured, and they were my third choice. Maybe you can score a similar bargain locally from Judo and BJJ tournament suppliers.
I'm no expert but I'd stay away from lesser mats. I've been on a few varieties of jigsaw mats without vinyl facing and they range from yucky to ghastly, and once dirty won't ever come clean again. Not to sound overly precious. It's fine. You train and basically forget they're there but it's just not something I'd want to step onto every day in my house. I just don't want to own something I don't like.
Also been on foldable mats with vinyl wrapping which had the vinyl separated from the foam. Not the end of the world but the little bit of slide from that unbonded vinyl would give a better martial artist than myself the shits, and I'd find it annoying for calisthenic and gymnastic endeavors. Also, they just weren't up to it for middle-aged takedown victims and I recall making an awfully noise after a teenage girl's drop seo nage sent me over flat backed. So not exactly an expert's voice of experience here but I'm really liking my Zebras and am pleased with the 'tatami' finish for purchase underfoot rather than dead smooth.
That's my 2.cents on mats but 1MR is going to be the dude to listen to if he has advice since he's logged thousands of hours more than me on them.
Fuuuck, it's past my bedtime, mate. Will check back tomorrow but if you want to DIY some stuff I can advise on framing, dry lining and ceilings, rendering, lime and hardwall skim finishes, painting, and finish carpentry.
Only last thing I meant to mention is consider what usage classification lets you in for. If calling it a potting shed rather than a living space saves you some sort of occupied square meters tax assessment. Electrical and plumbing regulations might be a headache if the building's use necessitates beefing up the existing house's mains. Of course, classifying it as a Granny Flat could do enough for your property value to make such things trivial. Sure you've thought all this stuff through already.
Subterranean areas really muffle shrieks and access to the most elevated height permitted by council regz is where you want the stargazing equipment.