Just taking a stab here, but I'm going to guess this is an older home with plaster walls and built on a grade level masonry foundation in a wet climate (UK?). My best guess would be that the garden doesn't grade away from the flat(s) and the foundation/slab aren't waterproofed. Masonry is essentially a sponge, and the only place for the water to go is up into the walls since you have impermeable flooring. Here in Seattle, a common retrofit for slab-on-grade homes is to trench a perimeter footing drain around the house to relieve hydraulic pressure under the slab since our water table comes right to the surface in areas during the winter. (Water wells will become artisan in winter)

They all seem clustered. What's the floorplan upstairs? It could be a leaking drain or condensation on pipes.

65% is high for me. I keep mine 45-50.

Is your foundation waterproofing good? How is the pitch heading away from it?

What is the actual chemical composition of the concrete? There was scandal here in Ireland where some builders skipped out on using the correct materials, used some shitty substitute and the houses nearly fell apart. It was pretty bad.

Check both foundation composition and building composition. Take samples from a few different spots and send em to a lab. If other houses around you were built by the same crowd then this is a possibility.

Best of luck.

That's water from outside.

A friend of mine had something like this and it was caused by two things: old leaking pipes in the floor, and the house being built in a hillside where the hill would direct rainwater towards the building, and the walls not being well waterproofed. They could fix the pipes but not the hillside, but it helped a lot.

How’s the gutters? Keep water away from your foundation and get your self a saniflow dehumidifier

Is this level of the house below grade? If so, you have drainage problems around the house, moisture is getting in from foundation walls and/or from below the subfloor.

If this is above ground level, you have possible roof leaks or pipe leaks seeping water through walls.

First thing to do is get a de-humidifier or two to combat the humidity retained inside. Then inspect exterior foundation for low spots, water drainage issues, cracks in foundation, etc.

A pro is worth the money here, even if just for diagnosis/consultation. They know the area, the building materials, the climate, etc. Very unlikely this is an unusual problem in your area.

Seems likely that it's a foundation/drainage issue. There's no easy fix for something like that, so it's well worth some money to figure it out definitely. To do otherwise is pennywise pound-foolish.

It could honestly be a number of different reasons, not enough ventilation on the roof, foundation, climate, age of the house, windows, doors,etc.I would get someone to come and take a look for a proper idea of what is going on

others have said some of these items but i'm just writing my thoughts out as they come to me so there will be some overlap but maybe my thoughts are helpful to you as well, good luck!

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1. 65% RH is probably higher than it should be. First thing to do is get a dehumidifier in there and keep it running so that RH is below 55%.
2. Does the kitchen have an exhaust fan that exits outside? (i'm guessing that it doesn't). Look into installing one but if this isn't possilbe then you'll just need to keep the dehumidifier running all the time
3. is there a bathroom upstairs? there could be some leaking into those walls but a person might expect the paint flaking to be seen everywhere along the wall, not just in those lower locations so this is unlikely
4. most likely, the walls are wicking water up from the ground. Check your downspouts to make sure all water is being pushed far away from the house. Is the ground properly sloping away from the house? Is the back "garden"/patio sloping away from the house? The next time it rains go outside and have a look and see where water is pooling or puddling. If it's in areas close to your house then you need to fix those spots asap.

I had nearly identical damp caused by two issues - damp coming from the roof down through the walls to the bottom of the house (yep) AND cavity wall to outside of house filling up with debris/crap and allowing damp to pass in at ground level from outside.

Paid someone to treat it by injecting a damp proof foam into cavity wall. Been fine since.

Get a damp proofer to come and investigate and they can tell you. It's money well spent.

See how the moisture is at the bottom of the walls? That only means it is coming from the top. Your issue can be related to roofing (or ground-drainage if you are below it)

When I got my house somebody blocked up all the air vents with sealant! Check they are all clear

your not drying your clothes inside are you?

Location?
Floor type? (Slab, crawlspace, basement, etc.)
What's above that area?
Estimated age of home?