Concrete thickness

What base/footings depth of concrete and hardcore will I need for garden steps built of paving slabs for treads and bricks for the risers?
The overall ground area is just over 1 sq mtr, dims will be aprox 1.2mtr wide x 0.9mtr front to back rising to a total height of 450mm, built against a terrace retaining wall of the same height.
Sub base is quite good, stable and flat.
Usage should be light.
All ideas welcomed.

6-8 inches will be fine

Got the concrete base /footing down at 75mm thick and now my lovely lady has changed her mind on the colour of bricks she wants which affects the thickness of brick I can get.
This now means I have to build the concrete up by 40-50mm. Am I ok to just lay more concrete on top of the base/first layer or does the base need some kind of prep or treatment first?

what sort of bricks are they???????

yes you can

Thanks for the reply Thermo.
The colour of the bricks was the prob. I was going to use Bradstone pitched walling bricks but now the only bricks I can source locally that are ok with my wife are `common concrete frogs` (name given by the local builders yard) which are yellowish/buff in colour and are a different size.
Only complication is while i`m only building steps with 2 treads I want the risers to be the same height (150 max with 450mm treads) with the top riser being the existing garden wall therefore the brick height is fairly important.
Thanks for the info. Will lay more concrete over w/end.