Why are some of my neighbor’s houses being torn down and rebuilt? In Brookhaven/East Sunnyside area.

Been noticing a lot of the older folk here are having new houses built out of nowhere. One day their old house is just demolished and in a month’s time a new one appears. Anybody know what this is about? I know it’s not just a random buy they decided to make since most of my neighbors are extreme penny pinchers but my dad only tells me the city helped them. Any input? Sounds really good as most of them are older folk.