I've been struggling with this lately. We moved into a new build when I was 11 and so the house was not run down, and I didn't realise how messy/dirty it was as a child myself. It was basically when my friends would come over and some of the less tactful ones would be shocked at certain things - I remember one day we were cooking sausages and one fell on the floor, so I just threw it back into the pan like "Oh it's fine the heat will kill it" and my friend was shocked and said "But the floor is disgusting - and you have animals running around over there!"
We did have animals living in the kitchen (pets, in hutches, though they were allowed to come out for a run around, and cats that had free reign of the house). I had never seen it that way - I thought my mum was a neat freak because she was always going on about cleaning - in hindsight, she was probably distressed by the state of our house. Her own health issues prevented her from keeping on top of it, and she was a single parent.
When I moved out of home, I initially continued to live in the messy/non-maintained way because that was normal to me and it didn't really occur to me for example that you should do things like wipe down counters/stove etc after use or wash up straight after a meal - my norm was to wait until something was intolerable and then tackle it. This caused arguments with my first live-in boyfriend but I did think he was being OTT. We ended up splitting up because of other reasons and then when I was living on my own with DS1 it came up again as a topic because my health visitor was concerned about the state of the house after visiting.
This went back and forth in discussion over a few months, because she'd explain that it was unacceptable but I think it hadn't occurred to her that I didn't really understand what exactly was bad about it. I would think OK, fair enough, it is a bit messy - and clean what I thought was right and then she would come back and be frustrated that nothing had changed. I found this very distressing because to me - it HAD changed! What helped remedy this in the end was that a social worker got involved and asked if I would like a copy of the form that they use to assess cleanliness levels of a house and I said yes please, and was given one.
Here is a copy of it - the version I had was more basic, and only scored at 0/1/2 (yes/somewhat/no) and only scored by the social worker, but the criteria were the same. I like the fact that this new version allows the parent and child to put their input in, too - this might have highlighted the fact that it seemed normal to me and it wasn't that I was neglecting it because I didn't care, it was that it didn't occur to me it was important.
The ones which surprised me/which I hadn't realised were a problem were the ones about "bits" being on the floor (every room of my mum's house has this), and washing up items being left to be washed until they were needed (I thought this was a totally reasonable approach). I knew that dirty kitchen surfaces and toys/clothes literally everywhere were a problem, but I thought it was OK if there were some parts which were dirty/messy as long as there was a space cleared to prep food, play etc. In hindsight, I also probably would have had a much higher threshold for "bad smell" compared to today.
Anyway - that's the backstory. What has happened since then is that I got diagnosed with ADHD and as part of this I learnt that I have difficulties sticking with routines, so I had a double whammy of having a baseline which is far too low, but also not even being able to stick to my baseline because of the ADHD. Having learnt strategies to deal with the ADHD I've been more able to maintain a better status of cleanliness and I've changed my baseline idea of what's normal.
DH also grew up in a non-maintained, messy house. Thinking back, I did visit his house as a teenager and it didn't register to me as messy or dirty. Because he doesn't have ADHD, he doesn't have issues with routines, but his baseline is similarly low to mine and we have both had to relearn this. However one thing I have learnt is that one huge key is getting unneeded things totally out of the house, whereas DH is still of a mind to either find a random home for things (making that area then look cluttered or be less usable leading to more mess/clutter) or shove entire collections of junk into a box/bag/pile and hide it in a cupboard
To be totally honest, I think our house is still dirtier than the average, though it is much better than it was before. We go through periods of it being better and then periods where it's a tip. But to visit my mum, or MIL, I'm immediately aware of this kind of general level of dust/mould/damp and grease and I do notice a difference between their houses, my own house and the houses of other people I visit. Mine is in between. Most people's houses I visit are nicer. My own mum's / MIL's are worse. My mum's house is now nearly 30 years old and the lack of maintenance shows quite clearly, whereas this of course wasn't an issue when we moved in. DH and I rent so we are not responsible for household maintenance but things like wall paint which has become scuffed - it takes us ages to register that it needs doing, let alone get around to buying the paint and doing it.