Knowing When to Expand your Base

This can depend on a number of factors, typically, you should expand after you mass your first wave of units and can cover any ground the expansion takes over by either stunning your opponent or defending a wave of attacks. There really is no limit to how much or how aggressively you expand assuming the expansions you place are intelligent and capable of being defended.

I think there are three kinds of expansions: (1) the advantage expansion, (2) the distraction expansion, and (3) the gamble expansion.

The Advantage Expansion--This is when you think you have the advantage over your opponent and he can't stop you from expanding. You can expand (relatively) safely because there is nothing he can do about it. Examples of this kind of expansion are the Zerg early expand, expanding because your opponent overspent on static defense, expanding after a successful hold or attack, or expanding after you have teched to a specific unit that your opponent can't counter yet.

The Distraction Expansion--This is when you expand while distracting your opponent. It is safe for you to expand because your opponent is distracted and won't know it. You distract your opponent with a direct assault (but be careful not to lose too many forces) or a harassment attack. Another example of this is expanding in response to an opponent's expansion, since you know they will be distracted by setting up their own defenses.

The Gamble Expansion--Sometimes you just have to expand to make things interesting. It is not safe for you to expand, but you're going to do it anyway because you came to play. Gamble expansions can be particularly effective if they are in places unlikely to be scouted.

Here are certain situations when you should expand:

• after holding off an attack

• after winning a fight, but not enough to finish the game.

• when you can't spend my money fast enough

• when you see spend your money easily and that my opponent has gotten one.

• when my opponent is turtling.
   

Expanding Tips as Protoss, Zerg, or Terran

Protoss:  It only takes one probe to quickly setup a protoss expansion, simply build a nexus, and if you think you'll need a defense, build a forge at your main base, and build a few pylons at your expansion along with a few photon cannons.  Photon cannons are great since they cover both air and ground.

Zerg:  If find it the easiest to expand with the zerg.  Early on the zerg are able to easily get extra minerals and will NEED to build a second hatchery anyways, so why not do it at an expansion site.  The only issues with this totally depend on the map you are playing on, if you can easly defend your expansion along with your main base (that there is only one choke point to both of your bases, you are set!  You should also get a second queen for your expansion or any other expasions you get to speed up the larvae process and to lay out more creep tumers.  Scout your enemy and decide on wether you should build a few sunken colonies or spore colonies, depending if they are going for air or ground.

Terran:  The terrans can get a bit tricky, because you will need more than one SCV to quickly build defenses.  One trick you can do is to build your command center inside your base and then when its complete fly it to your expansion site and quickly setup a defense and get it going quickly.  If it's well protected or you dont think your enemy will attack it often I would upgrade it to an orbital command center, however if you think it will get attacked, then upgrade it to a planetary fortress for extra protection.