Modifying and adding to an existing concrete base

If the original concrete isn't cracked lay the new part to the same depth, to key it into the old drill holes in the side of the old and bond in some stainless ties before you pour. That in it's self would probably do the job but there will be some cracks that form as it cures and shrinks (roughly 1%). I would be inclined to build your new garage on a couple of courses of engineering bricks then level it off with a couple of inches of screed when the concrete has had a couple of months to cure and dry out.

of course if you really want to go for comfort you could lay polystyrene over the base and pour a 4 inch reinforced slab over the top with underfloor heating pipes embedded.

I've just laid a shed base (not relevant) and either the bags of cement or the bags of ballast had a section on overlaying, mentioning 2" minimum required.



But that seems pretty thin to me, so i'd go more

That they are, but the materials used are not and i'm 99% certain I could not buy ballast in both shed or garage variety... :*

no but the thickness required between a bicycle and lawnmower or a car is vastly different

Once again, I mentioned the fact I was doing a shed to be irrelvant, the products purchased are standard and I was merely stating what it said on the bag.

I would be inclined to lay reinforcing mesh in both the new area and where the existing is, it should be in the middle of the thickness not laid on the bottom.

Defo lay the extra bit of slab then screed over the top of the two after the extra slab has had time to cure. Otherwise your asking for cracks imo!

Have you considered tiling the floor? That would be my choice for any workshop or garage nowadays. You can use a relatively average self leveling screed doing it that way as the tiles over the top provied your hard wearing surface.