If you are looking to install a home studio, there are a number of elements that should be considered before proceeding. Assuming you have either neighbours in the near vicinity, or indeed loved ones within the same property, one of the most important things to install is good quality soundproofing.
This will not only help to maintain harmony between you and your family and neighbours, but also help to prevent any bleed through of external sounds, like children playing, traffic noise or other sources of loud music, and prevent them from ending up mixed in with your recording.
Another factor to consider is acoustic treatment to deaden the sound within the studio and prevent any unwanted reverberations or other colouration of the recorded sound being caused by the studio space.
Assuming you don't have the budget for a full professional studio, and you're intending to use an untreated spare room, whilst you will never achieve a great enough level of isolation to contain the sound of a cranked studio monitor or a full drum kit, with a bit of compromising and following the tips below you can still significantly reduce the volume of sound travelling between rooms.
Sound Absorption
By adding a good, think carpet on top of an acoustic foam underlay you can make a huge difference to the amount of sound travelling through the floor and if you are looking to dampen the overall sound of the room hanging some old, heavy curtains or rugs a couple of inches out from the walls can make a big difference to the sound usually reflected by the walls.
Doors and Windows
As a general rule, doors and windows tend to be the worst offenders when it comes to letting sound in and out, but there are a couple of simple fixes that can make a world of difference. When looking to reduce the sound transmitted through windows, if you aren't too concerned about natural light, is to build a simple, but heavy, wooden shutter that can be fixed in place with sash window fasteners.
Due to their lower mass when compared to the surrounding walls, doors can be much more difficult to insulate effectively, however replacing a lightweight domestic internal door with the heaviest fire door you can find will yield some improvement, as will sealing any gaps between the door and its frame. Don't expect miracles however, it's not until you start adding double doors, for which there generally isn't enough space in a typical home studio, that you'll begin to see a massive improvement.
So to sum up there are a number of simple solutions that can make a reasonable difference to your home studio, however for best results the room will need to be fully treated by a soundproofing professional.