I’m exactly doing what you are expecting, I have bought cheap RF433mhz sensors and plugs like:
-door sensor
-PIR sensor
-smoke sensor
-wall switch
Added to that I have made some sensors myself based on DHT22.
All these sensors are going through an 433mhz arduino bidirectionnal MQTT gateway, you can do it also with an esp8266 gateway.
You have to be carefull with the antennas and power of the 433mhz components. I’m using fs1000A emitter + XD RF 5V receiver, FS1000A can be powered up to 12V to get more range, I’m using with it a small helicoidal antenna. The receiver is connected to a 3dbi antenna
A subject describe how to interface it with home assistant