"I know vinyl plank doesn't need a level floor, but is this dip too much?"

Absolutely way too much. Needs to be within an 1/8" over 10'. You may need to use self leveling compound. Make certain 3/8" underlay of ok, and that it is a solid core plywood or heals will punch thru.

it's a concrete foundation.

3/8 plywood is solid spruce.

thanks for the feedback!

Vinyl MUST HAVE flat. The dip you are talking about is TOO MUCH. The ideal fix is to resurface the subfloor (patch the divots with something so the new subfloor doesn't crack down in to the divots) and then lay the new subfloor. Once it is smooth, then the leveling can begin.

Or you skip the whole 'new subfloor' surface and pay to have the plywood skim coated. But that's a whole 'nother conversation.

Photos of what you have. And PLEASE listen to your installer. A proper 'flooring person' will know what to do. You must write into the contract the subfloor preparation. With a situation like yours, you should expect a quote for $2-$4/sf for subfloor prep. Installation of the vinyl will be extra.

There are a number of good leveling compounds on the market for floors; have a look at them, see which best suits your needs, and follow the instructions.

I agree it's worth having your foundation looked at, though, as that's quite a dip. You don't want to just compound over a serious problem, thus leaving it to fester and get worse. Having lived in a couple of older houses now where the problem wasn't a *problem* so much as just lazy or slapdash workmanship from the original builders, I can say it's not necessarily that something's deteriorating or gone wrong, it might just be that they didn't install the floor level to begin with...but it's best to know for sure!

Thanks for the feedback everyone. Here's a photo of the gap in the dining room. this is over a 5 foot span.

The area where it dips is where the old kitchen was, so it contained the stove, fridge, cabinets, underlayment and heavy, 1/2" tiles. Could the extra weight have made it dip over time?

the house was built in 1995. Our home inspection didn't reveal any foundation issues, so I assume we can rule that out.

As for the living room, the gap is about 1/2" within a 5 foot span.

Sorry for the abysmal photo... here's a better one!

If kitchen appliances and cabinets made your floor system "dip" over time you have more issues than the dip.

Ouch! That's 12mm over 1.5m! The allowance for most flooring products is 3mm over 3m. That is a BIG dip.

I would look to the joists underneath the figure out the issue. I'm going to guess you will need to sister the joists from below to get the dip to correct. And yes, if the 'heavy stuff' was sitting on a floor-joist system that was structurally inadequate (yet up to code...sigh) then things can 'sag'.

It's possible this is a foundation issue, but it is more likely a floor-joist issue.