I extended my double garage a few years ago to put in a two post lift.
Original double garage is 6m x 6m x 2.4 high to the bottom of the roof trusses and the trusses are 1 mtr high. It now has 100mm of fibreglass in all the walls and roof.
I have a 2.4mtr high roller door 5m wide and with 50mm of foam in it. Research since then has shown these doors are a waste of time insulation wise. A sectional door would be better insulated.
The garage extension is 6.3mtrs x 6 mtrs x 4mts high to the OBS boarded roof. The walls and roof have 150mm of fibreglass insulation.
Doing the numbers, polyurethane insulation is twice as good but I do not heat the garage for enough hours per year in most cases to cover the 3 times cost of that type of insulation.
In reality I should use a curtain between the workshop side of the garage and the parking side of the garage as I am heating the parking half of it for no good reason when I work there.
I have a 50 amp electric supply to the garage and we do not have mains gas. Therefore I use two 2kw electric heaters and they get the garage to say 11C on the coldest day which is great for working on cars.
If I did more hours in the garage, I would use an air to air electric heat pump which you can buy for say £750 but to date that has not worked out economic as I do not spend enough time in there.
If you want some calcs done re heating required and if the garage is some way from the house, cable size need just give me the details
