oliwoodings

I'm in Surrey, our current builders have retired but they recommended these guys and I've seen their work before: https://www.sheffbuild.com/

Got any pictures? Peeling suggests poor preparation of the surface before painting, not an issue with the paint. Is there fresh plaster underneath, and was it properly mist coated?

How close to the drain is the leak? From your photos, seems like it could be the installation of the drain and the way it's been sealed that's failing? I.e. in your original photos, could it be leaking around that blue tape? EPDM over the top sounds like a cheap tacky solution that will be hard to make attractive at all. Really the tiles need taking up and the waterproofing fixed properly, before a new finish is done (tiles/grp/EPDM).

I had other issues - I didn't realise Sonos no longer supports Google assistant with their newer speakers, so it was no longer worth it for me.

I got an era 100 the other week, it wouldn't stay connected to my ubiquiti network. I sent it back.

He wants to put a ride on mower in it, not sure I'd want that weight on a wooden shed floor not supported all the way underneath? £1100 including labour seems reasonable to me. I paid £2k for a 6.5 x 5.5m 100mm reinforced slab with 100mm MOT underneath a couple years ago

Another thought - you seem certain it's the balcony floor that's causing the leak downstairs, but could it also be the doors/windows? They might not be properly sealed, for example. You mentioned it only happens with high winds, which could support this theory. Do you have any pictures of the leak from downstairs?

How long ago was this done? If it's within a year I think you'd be well within your rights to get them back to fix it.

You can't really think of the tiles themselves as a perfect waterproofing layer. Grout fails over time, surface sealants wear out etc. Your flat roof substrate underneath the tiles is what needs to be bulletproof. If water is coming through then it suggests an issue with that substrate - either it's failed, or been installed poorly leaving a route for water to get through. Do you know what's underneath the tiles? Is it too late to get whoever did the work back to fix the issue?

Yes I did the foundations level with the ground. Only mistake I made was oversizing the slab compared to the wall perimeter due to changes in layout. This meant bits of my slab stick out under the walls, creating drainage issues. So, just make sure the slab is sized correctly and built correctly!

I built a garden building for my wife's business, she uses it for 2-4 hours a day max all year round. I went waaaaaay simpler than your architect, and did it all within PD so no planning required. For floor I did 100mm reinforced raft over 100mm MOT, and then floated 100mm PIR over the top with floating subfloor on that. The walls sit directly on the concrete, with the PIR inside the perimeter. 2x4s infilled with 100mm PIR. Roof i did a hybrid design to save on headspace, which is fine from a condensation perspective given it's low occupancy. 2x7s with 100mm PIR pushed up to the top, leaving a 50mm service void, then a vapour barrier across the underside before PB. 18mm roof deck with EPDM. I got all the PIR from a b-grade insulation reseller to save some money - perfectly fine for a garden building. Take some of the money saved on the over specced bollocks and put it into an A2A heat pump. Then you'll be comfortable the entire year round. Cost us £1600 for a split multi-room AC install (our building is split into two rooms), best choice we made.

I don't see why it would be an issue - I assume you'll be attaching a wall plate down first on top of the blocks and the sips will sit on the plate?

Couldn't you use 100mm blocks and take advantage of the overhang by positioning it on the inside, then running your floating PIR under the walls to reduce cold bridging?

Wouldn't a non permeable dpm be better than permiable geotextile membrane if rockwool is gonna sit against it? I'd be worried about moisture wicking up into the rockwool through the membrane personally.

I don't mean to be harsh, but don't you think you kinda brought this second mess on yourself? You actively chose to get back a tradesperson who you already know made a potentially life-threatening mistake and he wasn't even able to fix it when told about it originally! What made you think his second-fix plumbing in your bathroom would be any better? If he didn't provide a quote, what did he agree to? What do you have in writing? Did you take detailed photographic evidence of his mistakes before you fixed them? If you've got pics, ask him for an itemised bill, and then you can prove that he didn't actually fulfill those things properly and therefore you're not paying. But to be brutally honest, I think you partially brought this on yourself and might need to chalk it up as not-too-expensive life lesson.