Help me get started

I'm looking for some help with lawn prep for seed. I have been reading threads all morning and I have found a wealth of information but I wanted to post anyway.

My garden measures roughly 14m length by 13m width with paving slabs along the east side of the house and across the back. My plan is to deck over the paving slabs along the back, 4.8 x 3.5m, use decorative stone for along the south side of the house (narrow dead space) and then seed the rest.

My issue is with the prep, the house is a new build and the builders put 2" of sand across the whole garden and its been sat like that for about 4 months. I have scraped off sand where the decking is going to reveal the following:



Plus, at a depth of about 200-250mm, I have dug through the ground to find type 1, this was in 2 places, both within the area of decking.

I need to reduce the level of the ground by 150mm within the decking area but for the rest of the garden, I don't know what to do. I plan to hire a tiller next weekend, go over the garden and add some compost, leave it for a week and then I guess I'll need to add topsoil? Rake to level and then seed?