When I moved into this flat I found a few problems that I am glad I fixed promptly.
The first was the front door. It was not sealed properly, so it let a breeze in, making the place quite chilly.
$20 weatherstripping made a big difference. It took 10 mins to install and my hallway no-longer has a draft!
Second was my balcony door. It wasn't aligned with the frame, so there was a gap at the top-right. Again, it was letting cold air blow into the living room. 1/