You really need to consult with a landlord/tenant attorney in your area. Closing off a master bedroom and renting it out as an additional apartment may violate your local zoning ordinances. And, if your lease is for the entire house, then it seems to me that the landlord cannot simply take back some of the space you are renting in order to rent it out for someone else.
Generally, if your dog bites someone you may be liable depending on the circumstances. In some states, and recently in Maryland there's been a debate on whether landlords should be 'strictly liable' for injuries caused by a dog such as a pit bull. Thus you want to be careful, as I don't know if Florida has similar type laws.
DISCLAIMER: Brandy A. Peeples is licensed to practice law in the State of Maryland. This answer is being provided for informational purposes only and the laws of your jurisdiction may differ. This answer based on general legal principles and is not intended for the purpose of providing specific legal advice or opinions. Under no circumstances does this answer constitute the establishment of an attorney-client relationship. For legal advice relating to your specific situation, I strongly urge you to consult with an attorney in your area. NO COMMUNICATIONS WITH ME ARE TO BE CONSTRUED AS ARISING FROM AN ATTORNEY-CLIENT RELATIONSHIP AND NO ATTORNEY-CLIENT RELATIONSHIP WILL BE ESTABLISHED WITH ME UNLESS I HAVE EXPRESSLY AGREED TO UNDERTAKE YOUR REPRESENTATION, WHICH INCLUDES THE EXECUTION OF A WRITTEN AGREEMENT OF RETAINER.
Contact a landlord-tenant attorney immediately to address your rights under the lease. The attorney will probably have to review the lease. Florida has strict liability for dog bites. Posting Bad Dog signs helps and the landlord will probably have some liability but you will probably get sued along with the landlord in any dog bite claim.
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"I have dogs that can and will bite strangers inside of "their" yard. My landlord knew this before we moved in."
If the bite occurs on the property, both you AND your landlord can be liable. You can limit your liability by posting certain signs in accord with the law and taking certain precautions. However, this will not completely inssulate you. In Florida, if your dog bites, you are almost always liable!
Time to move elswhere...
I would strongly recommend speaking with a landlord/tenant attorney. Although both you and the landlord will most likely be sued if your dog should bite, there are other problems that may need to be addressed. For example, if you we're previously renting the entire space, the landlord may not be able to quarantine a section of the house without repercussion with regard to improper partial evictions or even possibly constructive eviction.
In short, you need to take all your documents to an attorney before something happens and access your rights.