Legality of changing a spare bedroom to a bathroom

If your house is not listed or conservation all you need to do is involve building control. It may be in your interest to just let the council know that you have 1 bedroom less, I am not sure if it will reduce your council tax

Council tax is usually measured against the value of the property on a certain date, so you should talk to the council to see if this is going to be affected. You must talk to the planners regarding building regs etc as you might need to be a qualified plumber/electrician as you will be working in a bathroom. For the sake of a phone call, you will have your mind set at ease and stay legal. Sadly the days of being capable are over, especially where electrics and gas are concerned. Everyone wants a bit of paper now, even though you might be the plumber in the world. Without the bit of paper to say you have been through the training they want you to have gone through, it doesn't matter. I can plumb and electric my heart out, but it still needs to be signed off by some oik with a bit of paper who has not been alive as long as I have spent fitting pipes together.