You may own one house on any of the main shards; in addition you may own one house on either of the special rule-set shards, Siege Perilous or Mugen. Also the test shard is entirely seperate, you may safely test a house design there without endangering your existing home.
I am sure everyone has seen an increase in the decay rate of items on the ground, houses, and boats. This is due to a fix we had to put in for a shard crash. Due to the fact that the house decay does not give players enough time to move their items from one house to another, we are going to be putting in another patch to change this. Players will have 5 days for a house to decay.