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Naturally, the word "cave" is very loosely used in the case of developing a man cave. It can be the attic, an unused bathroom, the shed out the back, the garage, or the basement. Or anywhere else that's spare, has adequate elbow room, and is left currently unused or pretty much abandoned.  Build a shed or dig out a basement if your options are looking thin. . Having a man cave in the house may require delicate household negotiations and compromises. You may have to agree to something in exchange for your man cave been allowed into the house. If your man cave is using up the only relaxation space, no one's going to end up happy. Try offering one of the following in exchange, if your partner or housemate is interested:  Setting up another room as a sewing room, exercise room, sauna and spa, playroom, model collection space, or astronomy observatory. Promising a certain number of hours each month on household repairs, odd jobs, or chores. Arranging regular dates or family time. Man caves often start in corners of the house without insulation. Make your man cave warmer and more sound-proof by installing fiberglass or other insulation material between the wall studs. Unless your cave is in a stand-alone shed, or you're living alone, put a priority on sound proofing when choosing insulation material. You don't want your midnight sports or poker game to be interrupted by a tired spouse or neighbor. If you've been relegated to the dingiest, smallest room in the house, bright colors will give a sense of more light coming in, and more space. On the other hand, dark colors are good for home theater setups, or a relaxed bar-like atmosphere. If you couldn't care less and just think a wall is a wall, congratulations. Skipping this step is the first benefit of having a space where you get to make the decisions. Install recessed lighting or a light on a dimmer switch if you want to watch movies in your man cave. If your man cave has windows, put up blackout curtains so you can pull them closed and game at three in the afternoon without the outside world butting in.

Summary:
Choose the space. Be prepared to negotiate Insulate and sound-proof the room. Paint or wallpaper the room (optional). Add lighting or curtains if needed.