Dealing with sinking spot in lawn

Howdy
I haver noticed a dip in the center of our lawn. It's gotten worse over the last few years. I'd say it's roughly 6m in diamteer and has sunken maybe a foot at the deepest part. The house is 20 years old and my fear is the builders left a load of rubbish out back and just covered it over with topsoil. Over the years the rusbbish has degraded leaving the aforementioned. I could be wrong but tbhis is what i suspect.
I don't have the access to get a mini digger in so I'm looking for advice on how to recify the issue. I suspect just filling it in with more topsoil may not be the solution?

Without digging it up to see what the problem is, it will be very difficult for anyone to offer a solution

If it's just old site rubbish that has decayed or compacted over time, then just adding some top-soil and levelling will resolve it.

Any water pipes or drains running through that area that you're aware of? A leak could cause this. If you're curious and fancy some exercise, grab a shovel, carefully remove the sod and put to one side, then dig.

I don''t believe theres any services running underneath at all.

If not an underground stream, then it's most likely just settlement that will stop in time.

Slice off a section of turf in the centre with a sharp spade and park to one side. Get a good old pickaxe and investigate. If nothing worrying, remove the topsoil, add gravel or stones to raise height back up, replace topsoil, replace turf, water well, repeat over the entire sunken area.

As Leo says, it could be a problem but probably isn't. While you can raise height of grass by just adding sand on top, I think for your own peace of mind you need to check what is underneath.

OK folks thanks for all your replies. I'll tackle this on my days off and report back.