Some people will tell you that ALL batteries need to be in special boxes with vents and fans etc. etc. Others will tell you that only FLA batteries need special housing. Still others will tell you that they've lived with FLA batteries under the head of their bed for years, open to the inside of the van with nary a problem.
Me I have 4 FLA 6 V golf cart batteries where, if I wanted to, I could touch them while laying in bed. I don't worry about it.
BTW, it's not as easy as 'adding more batteries as I go along'. All the batteries in a bank need to be of the same age, etc.. Pairing older batteries with new batteries will have the new batteries acting as it they were old in no time flat. If you need to add more batteries to a bank then you'll be replacing them all at the same time.
If you're talking about 'storing' batteries as opposed to 'using' batteries, you want to make sure they're fully charged and then put a battery minder system in place, either solar or shoreline. You do not want to store batteries without looking after them.