If the place you are in is already built poorly, then the it is neither "very easily" or cheaply fixed.

Try convincing land-leeches to do anything but the bare minimum cheapest option and you'll be out on the street in no time.

sweden has a lot of "commie blocks" built around the 60's, which are generally basically solid concrete, and straight up the only time i hear my neighbours is if they drop heavy stuff directly onto the floor or if we both have a window open.

These buildings were made specifically to be cheap housing, and yet they seem to be some of the better housing available in the world, fucking wild. I maintain that our commie block areas are some of the best places to live within the country, you get hilariously cheap rent, car-light surroundings, generally very decent public transport connections, and it's not unusual for them to effectively be the situation depicted in OP's image, some example areas being Bergsjön, Fisksätra, and Jonsered, the latter of which is wonderful because it's effectively a small town consisting solely of apartment buildings.

I lived in a condo that had thick stone walls built after WW2

It was still shit and I will do everything in my life to never have to live in these giant human hutches.

My house is right next to an actual forest. I can hop out of my vegetable garden and be hiking on a moment's notice.