Posted: 09/12/2016 at 14:35
It depends on what is hiding under the pebbles and membrane. I think you need to check that first but, in order to remove the membrane, you will displace the pebbles anyway so why not pile them up in bags as you go and offer them on Freebay to anyone who can collect.
I think just adding topsoil on top of pebbles is asking for trouble later on and will also mean you need some sort of retaining mechanism - bricks, tiles, logs - to hold the soil in place - more expense and maintenance..
If you want your lawn to grow well and not suffer from either drought or poor drainage, I would clear the pebbles completely then push a good string gardening fork into the soil as deep as possible and every few inches then wiggle it about to create holes. Pour on sharp sand - not builders' - and sweep across the surface so it sinks into those holes to provide drainage. Then pour on the top soil to the level required, rake sooth, trample lightly with your heels, rake again and then sow seed.
The best times to sow seed are April/May and September when the temperatures and moisture levels are conducive to good germination and growth. You need to pick a seed mix to suit your situation which will depend on whether those tall trees cast shade all day or not plus how much wear and tear it will get.
There's more info here from the RHS - https://www.rhs.org.uk/advice/profile?PID=424
You can also lay turves but that is more expensive by far than a seeded lawn.