I used a much less expensive method, which I did myself (since you obviously don't want others knowing about your hidden room).
I had a rectangular room (used to be the kitchen) that I was going to remodel into a office/spare bedroom. In order to put a closet in I had to build a new wall on one end of the room. While planning this, I got the idea to slightly extend the depth of that wall and build a hidden grow room. From the office/bedroom all you see is a regular wall with a typical (7 feet wide by 3 feet deep)bedroom closet in it. BUT, push the right spot on the back wall of the closet and a panel/door will silently open (I used spring loaded gate hinges so the door stays snug and shuts firmly on its own, and use accent strips/trim to conceal any seams). Go through the door and you are in a small room that is also about 3'x7' with a 12 foot ceiling. On either end of the room is a zippered heavy plastic panel that serves as doors. Behind each is a 4'x6' grow room equiped with a 600-watt digital HPS/MH lamp, ventilated via a pair of 4" filtered air inlets that pull cool air from under the house. this air is pulled into the room and exits via a 6" ventilated hood/reflector. it exits the hood and is blown out into the attic (a portion that is totally inaccessible) through a 590cfm in-line fan. The 2 grow rooms are completely lined with mirrored mylar on the walls and the floor and ceiling are painted white, plus they each have a 16" oscillating wall-mounted fan to help circulation.
Exit either of the grow rooms and go back into the first 3'x7' room and look up and you'll see the 6" flex ducts feeding into the in-line fan and exiting through the ceiling. Climb the ladder in there to my "second level" and I have another grow area. Actually it is 3 areas. the middle one (actually the top of the closet ceiling) is equipped with a propogation set up for cloning/sprouting seeds, peat cubes, extra HPS and MH bulbs, cloning gel, watering jugs, etc. The other 2 areas (the tops of the growing rooms) are equipped with an assload of CFL's with reflectors, and are lined with reflective foam insulation and reflective mylar. I use these for most of vegging (usually until the girls are about 14" tall), but then I'll do the last few weeks of veg under a 600w MH "downstairs".
Anyways, the setup is virtually undetectable by humans unless someone shows it to you. I spent about $700 on materials, the rest was my labor. I now have a VERY nice office with a closet full of stuff --- that hides my little garden. It's pretty much impossible to do something like this unless you own your house. But if you do, it's definitely worth pursuing. Get a good buzz and get creative with it! I already know of a few improvements I will make when I build my next house --- which will have a purpose-built concealed grow room from the start.
