I want to ask my parents something... HELP?

I am going to be 14 in 3 weeks and well, I want more privacy. I have my room in the room next to the basement and well, lets just say that the bathroom is the next story up. I want to ask my parents that for my birthday, if they could build me a small bathroom down here in the basement/my room. My room is fairly large and all I would need is a toilet and a sink... It's just hard when it's 3AM and I have to trudge up stairs. Or let's just say it's "that time of the month" and i don't want my parents to see me go to the bathroom, every so often! I just want some privacy. Sorry this is soooo long but, I really feel mature enough, and all I want it to just have my own space. Anyone know how or what I should say to my parents. Please be thorough I really want this! Thanks in advance.