Roof joists good or bad

Sounds it ,,,,usually sat on the inner wall plate ..

It doesn’t sound ideal, but without seeing exactly what is happening- The best advice that I can give is - if you are concerned speak to the builder - you and your builder should have a relationship where you feel comfortable to discuss any concerns that you do have.

I would expect a 4x2in timber wall plate to be set on top of the internal blockwork and the joists to be fixed to that. You don’t mention the roof but I am assuming that it is flat and therefore the joist would continue over the outside brickwork, cut flush for a fascure or longer if there is to be a soffit. Either way the joist fixings should be down on to the inner wall plate which is also strapped down onto the internal blockwork. The difficulty here is approaching the builder because one way or another he is going to defend his method of carry out the work. Your in between a rock and a hard place. As it’s a garden building, I’m not sure building control would get involved, the inspectors are good people and might pop out to advise, which may help with discussions with your builder, it’ll only take a phone call but other than that, it’s a case of bite the bullet and approach your builder and ask him to explain his methods

The inner wall takes all the load from the roof, and floors above if present, not the outer wall. In my opinion what has been done is wrong and should be corrected. Are there drawings available for the builder and any structural calculations to support the design? Is the builder following formal instruction from another body ie the designer, architect or architectural technician? Is this to be approved by Building Control - it should be? If they are involved it should be picked up by them and raised as an issue unless drawings indicate otherwise.

Totally wrong joists should sit on a wallpaper on the inner wall and ceiling joists tie the two sides together if it is a pitched roof if it is a flat roof the joists should be big enough to span the gap