Can I put new turf straight over existing turf to bring the level up?

[Erebus]

Our oblong lawn obviously had, at one stage, a wide garden bed on one side, perhaps a 1/3 of the width of the now-lawn. Over the years, that section has subsided a good 2-3". There is a distinct step between the heights. I would like to level the whole thing. Can I just lay new turf straight onto the lower section (on top a thin layer of topsoil, maybe)? Will it survive (watered, etc, of course!)?

I'd think your problem would come as the turf rotted down, the level will fall again.
Can you lift the current turf and back fill with top soil / compost?

Good point, next- rethink.

I imagine I'll just end up throwing money at the 'problem' .

Just fill with topsoil, firm down well so that the level is correct, and rake in grass seed. Water well and Bob's your uncle.

Yes, seed is the way to go.

How long does seed take to establish, ooi?

I think it's about 5 weeks before you cut it.