Sunken garden

My garden has sunk up too 9 inches in places. The existing grass has a lot of weeds in it so I want to build it up and returf. Would I have to remove old grass first or can I buil it up then put a layer of top soil on and returf. Thank you.
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Do you live in an old mining area by any chance, or maybe, judging by your avatar, you are in Australia?
No garden has just sunk over the years not a mineing area. I'm in England but Avatar upside down tried to change it but failed miserably ha ha
If you send a private message to NoraGW she’ll turn it round for you.
maybe the previous tennant dug the ground a lot and it was very loose, just compacted over the years.