I just got the iPhone 5s, Does iMessage use my data when I'm connected to Wi-Fi?

I only have 5gb of data a month. Will iMessage use my data up when connected to Wi-Fi?
also, when i leave the house and I'm no longer connected to Wi-Fi, will iMessage cost me?
I don't want to go over my bill!

No it doesn't, esp in UK when you are connected to your Wi-Fi it uses your home internet rather your mobile internet. However you can double check this thing from your service provider as well.