base for garage

Hi all,
I want to extend a garage and thought at first it would be a simple task of removing one wall getting some more concrete panels to extend the back wall, concrete the base, and put the wall back, however ive realized the concrete floor that the garage currently sits on is out of level by around 30mm
Pics attached: you can see were im at, Ive dug out the base to around 250mm - 300mm
I will fill in the base with around 150mm hardcore then 100mm concrete. however because the current floor is not level, I guess i will have to remove the other walls and level the floor. Is it ok to run the concrete right across the existing base? its just that its going to be a rather thin lair? or is something else required?
I have never done something like this before... Can anyone give me advice on whats best here?
Thanks
legepe

I'd pour the new concrete to the highest point on the old base. Then level the old base with suitable self-levelling compound. Don't use thin layers of concrete, it will come off.

ok thanks for that ivixor..
am i ok with 150mm hardcore and 100mm concrete, should i put dpm in? if I decide to build a brick garage, is it ok to build on top of a self leveling compound?
Also, the concrete slab to the right in the pics.. will this need to be broken up or can i concrete over that. at a guess it is around 2" lower than the highest point on the left so it will have 2" of concrete over it?

You would build the garage off new strip foundations not on the edge of the slab.

Hardcore, sand blinding 25mm, 1200 guage dpm then your concrete.

alastairreid said:

Hardcore, sand blinding 25mm, 1200 guage dpm then your concrete.

Click to expand...

I would do as you say with foundations, then would it be an option to raise the floor level meaning that the thinnest part of concrete would be 75mm which would be the right hand side of existing floor, everywhere else would be thicker up to 105mm, is this acceptable? see pic
I could still put dpm in.. NO?
Do i put in side-boards where concrete will be up to before putting hardcore and sand in or is this done after?

Yes 75mm at a minimum but would get fibres added to the crete.

https://www.cemex.co.uk/fibre-concrete.aspx