oliwoodings

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.

Check out sun tunnels for getting natural light into the garage offices.

What's underneath the osb? Could you just rip it up and replace it? Alternatively do you have space to just overboard it with 12mm ply perpendicular to the current subfloor?

You're wise to aim for this, because then the person knocking the wall down and making good is accountable for thinking ahead for the kitchen stage.

If you've got an architect, why aren't they just giving you a design that fits within the 2.5m restriction?

That doesn't say anything about the 2.5m height restriction though. What advice have you found that states you don't need to include the 150mm upstand inside the 2.5m?

Ah okay makes a bit more sense! I'm still a bit confused about where these 150mm upstands are in your drawings though - in the last picture have you removed the outer block skin?

Why... I'm just trying to help keep the forum quality high? Op has repeatedly been creating posts with the title as their username despite others already asking them not to. I figured I'd help them learn how to use it properly with a quick screenshot?

When creating a post it really does help if you give it a proper title, like you would an email subject:

No in your case it's to the underside of the felt.

Did you intend to post this thread twice? https://forum.buildhub.org.uk/topic/38066-garage-insulation/#comment-548989

Some pictures would definitely help give advice here if possible. Insulating inside and out shouldn't fundamentally create condensation issues as long as you have an appropriate vapour barrier on the inside and ventilation behind the cladding on the outside. If you insulate externally do you have enough soffit depth so that the existing roof covering continues to extend sufficiently over the edge of the new finished wall surface? Bear in mind that almost all cladding & rendering options will require 25mm battens underneath for ventilation. If you turn it into a bedroom, I believe you will at the very least need to meet building regs and get BC sign off. May even need planning, although I'm not clued up on this area myself. If they are just going to sleep in it very occasionally then might be best to just do it as an office, and oh no you accidentally put a sofa bed in it whoopsie.