Karndean flooring - preparation work and smoothing compound

The process sounds right you could always get another quote done you may get it cheaper but it is a large area

Yes, you should question it. You should always do your own research. Ask the contractor what self levelling compound product they are suggesting to use and then see what the manufacturer recommends with regards to laying thickness. You may find that you can lay this 6mm layer all in one go. The final skim coat is really to cover any gaps between the layers of ply and also the join between ply and concrete as well as the sunken screw heads in the ply. 3mm sounds like a lot, its literally supposed to be a skim, but again check with the products manufacturers and see if there are alternative products available. Always do your own research, the information is out there and easily accessible. With LVT it is important to have as smooth a floor finish as possible, this cannot be overstated. Plenty of customers think they have a level and smooth floor but when it comes to it there are levels and then there are levels, a professional floor laying will almost always insist on prepping the floor first to their satisfaction. That way they can be sure, if something happens to the floor, what the reason behind the issue is. Most customers don’t want to pay for this service but it is necessary. However, that said, you need to make sure you’re not being ripped off too.

Unless the t&g flooring is badly damaged and the plywood is being added to strengthen it then its not required. A sealer should be applied first to the t&g area and then a final floor leveller compound over the whole lot. If the t&g is damaged then replace the boards. I suggest you get a couple more quotes.

The process is correct that the floor layer explained however that is a very steep price just to prepare. You would also require sealing the screed floor first before putting the smoothing compound down otherwise you may risk the smoothing compound blowing where it may not have adhered to correctly...

In reference to the process, that is correct. The concrete will require a self levelling latex (what the installer is referring to a smoothing compound) and the chipboard will need a plywood over the top. It is best to apply a primer over any area that will be levelled. That being said, it may not have to be 3mm at a time as some manufacturers of new levelling compounds state they can be used for much thicker depths, however due to the m2 it is always best to speak to the manufacture technical team as you may be more susceptible to cracks if too thick. Equally, 3 layers of 3mm latex isnt ideal either. It can be argued that depending on the size of the area where the chipboard is that this doesn't need to be latexed over. Pouring the levelling compound over the concreted area (be it 1 or 2 layers) and then applying a feather edge over the top of joints between ply and latex will suffice (this can also be applied over the entire floor after the latex is poured and dried) It is worth noting that although other things can be done to ensure level floors as mentioned in above comments, LVT flooring manufacturers are very particular and stipulate strict processes for subfloor prep. Karndean and Amtico in particular. Of course this is only if the use of warranty or claim is required - however if their processes are not followed, their warranties are void on inspection. They do stipulate a minimum of 6mm flooring grade plywood. The processes isn't a cheap one when done correctly, particularly when ensuring not only the levelness of the subfloor but the strength of it. That being said, the price seems excessive - its always best to get a couple of quote for comparison. Hope this helps.