Re-turf a garden that is soft/sinking?

Hi all,

I'm planning to sort out my back garden as it is weed infested!

Here's the real problem tho - the ground is so soft & unlevel. If you stand in one place just for a minute, you start to sink!

The house is a newish build (3 years), so I'm hoping the garden wasnt done properly?

Now this is what I'm planning doing:

1. sprayed the whole garden with weed killer (it will kill everything) - have this this today.

1. wait a couple of weeks for everything to die

2. remove the top layer of dead weeds/grass

1. Rotovate the ground

2. Lay some new soil if needed

1. Rake until soil is fine & level

2. Lay new turf

Is this a good plan? BUT how do I address the issue of the soft/sinking ground?

Thanks a lot!