If you are in Europe, you can also buy the ESP8266 from Olimex with relay, box and eventual lipo plug. You then just have to flash your code on it. Only downside is they are not properly shielded electromagnetically so they can only be sold as Kit for experienced users (as certification costs would dominate the price). Also, if your house happens to burn, you can be pretty sure the insurance companies will pin it on your uncertified DIY appliances.
I don't know of actual precedent, but there are numerous examples of lipo house fires (which aren't fully covered). And semi related personal one-datapoint anecdote I've experienced water damage and robberies in the past and they were a lot more prompt to try to find loopholes to not have to pay. I can't afford to set a precedent, so I stick to fused low voltage off the shelf products.
Use an off the shelf wireless plug in switch and a Telldus transmitter then you don't do any high voltage or high current wiring at least if you are controlling a standalone heater.
