Well the Zerg do have a cheaper "Town Hall" than everyone else, though it still takes the same amount of time to build. It should be easy to control an area with a swarm of hydralisks, especially after you had a successful attack. I usually try to expand at the same time as I attack, making it harder for my opponent to attack my expansion while it is being set up. You should build at least two hatcheries at your second base, and make sure that it is close to your main base, since you can get an instant supply of drones from your 2-3 hatcheries at your main base. You should build a detector of some sort at your new base as well, since the number of cloaked units in the game has just increased. Prefferably use sunkens and spore colonies, since overlords are easy to kill, and you need overlords to transport and scout.
So after attacking, you should have done a great deal of damage, you should expand at this point since your opponent can't attack and doesn't know of your expansion. Even if you can't do any damage, make sure to run away, since losing Zerg units is an easy task, but keep them around near an overlord to start containing your enemy.
Now another way to expand is build an early second hatchery at an expansion, and then a spawning pool to get sunken colonies. You should only do this on maps like, Lost temple, Ashrigo, Bone Canyon, Winter Conquest, etc., any map with a close second base. Now you should immediately start building sunkens at the base, to block off any attacks from your enemy. This does work if you can prefect the build order.
Now when you have your second base running, build more hatcheries at your main base. This is only because the Zerg expansions are easy to take out. Fueling the second base using the main base's hatcheries is a good idea. Try to take some time to take the larvae from your 3 hatcheries in your main base and build drones, once they are finished run them up to the new base, and you have instant income. You can then go back to producing other units, and use the expansion's hatchery to build drones. Now build more units and expand to the next expansion closest to you.
On island maps, it's hard to setup expansions, because you build only 1 hatchery most of the time, and then you are slowed down even more by building sunken colonies and spore colonies. Now to get around this you have two ways. First you can build a nydus canal, but that's later in the game. The other is to build drones from your main hatcheries and transport the drones over like the other races. This is something that no one does, yet I can't see why. It's the fastest way to get a base up for the Zerg, especially on an island map.
The Zerg expansions are meant to power the main base, not be a self reliant base itself. So try to keep most of your hatcheries in your main base, and transfer drones to expansions.
