Basement flats are great for so many reasons. They usually have a separate entrance and their own patio plus most of them come with storage rooms, too. They are also cheap. You can get a decent 100 sqm apartment for the price of a 60 sqm one. For this same reason, if you need a well located flat, this is the one to go for. In central London with sky high rents and immense competition the only way of getting a well situated place for a reasonable price is to go down under.

On the other hand, the  common belief is that these flats are dark, damp and smell of mold. Most people try to avoid them at all costs. I admit, there are basement flats that are damp, dark and covered in mold, but this is not the target market.
To know what to look for, you have to know what to look for :). The main enemy of these properties is water. Insulation should solve the problem, but most horror stories you can hear prove the opposite.

Fact: a basement flat needs to be very well insulated.

Water from surrounding soil can make the walls soaking wet, and where’s water, there will be mold.
So why are there dozens of people complaining about wet walls and mold in flats that just have been thoroughly waterproofed?
Well, what happens if you seal a waterproof box completely with an evaporator inside?
All the water that can not escape will be on the walls once again.

But where is the water coming from?

Basically? You. Humidity is formed by breathing, cooking, washing, drying, having a bath, a shower.. Simply said, by living. So if a flat is well-insulated, it will keep the water out from the outside, but also the water in that is formed inside. Both result in wet walls.

Is there a solution for humidity?

Ventilation. Fresh air.
Being under ground kind of limits the size and quantity of windows, so getting fresh air in can pose a challenge. Any fans that are connected to the outside world can be huge help. It is a necessity in rooms where the most humidity is produced, such as the kitchen and the bathroom. During summer having all possible windows open for a few hours is a huge help.
A great solution can be a dehumidifier. It filters the air and gets the water out of it. These circulate the air inside, meaning windows don’t have to be open and you don’t end up heating the street.
Mold needs humidity levels above 75% for 72 hours, so if you make sure to have ventilation sorted out, mold will stay away.

My advice

I live in a basement flat (a duplex really, as I have rooms on the ground floor, too) so I thought sharing my daily routine could help. When we moved in we soon ended up having a whole wall completely soaked and it took me quite some time and research to dry them out and it still needs constant attention to make it stay this way.
(Buy a higrometer!)
Every morning when we get up we open all windows for at least 15 minutes downstairs. If it is warm outside we open all windows as soon as we get home and only close them when it starts to get dark outside. Every time we cook or take a bath the fan is switched on. And we also open the windows after meal or shower time.
In countries like England and Belgium sometimes simply opening windows can not help as the humidity outside is just as high. On cold and/or humid days we still open the windows for 10 minutes in the morning and in the afternoon, but before going to bed we also switch on the dehumidifier for 1-2 hours.
In the beginning, when your walls are very wet, try to have the dehumidifier on for at least 6 hours a day until the water clears off.

There are weeks when the humidity levels are lower outside and I only need to open windows to have humidity under control and there are days when the machine is needed. I learned to balance it and with a bit of attention we live in a mold-free, dry-walled basement flat. Which has the location, the size and the looks for half price. 

These are the pics I took of the apartment after the first visit with the estate agent. It was love at first sight. The kitchen and the living area are on the ground floor, 2 rooms and the bathroom are on the basement level.