If I am renegotiating a business lease and the property management firm is asking for my personal tax and financial information, is this legal and/or required?

Is it legal? Yes. A prospective landlord may request any documentation from its prospective tenants that it wants, including personal finances in regard to a commercial lease. Remember: they are not obligated to rent to you, and you are not obligated to rent from them--they can ask for any documenation they feel would be helpful, and you can walk away from this potential landlord if you don't like what they are asking.

Is it required? If this landlord wants it, it is required. They do not have to rent to you unless you satisfy them that you are a desirable tenant. If you don't comply with their requirements, they won't enter into a lease.

There are a number of reasons they may wish to see your financials, by the way:

1) If your business is sole proprietorship or partnership (not an LLC or corporation), you and the business are legally and financially the same, even if you don't personally see it that way.

2) For smaller, newer, or otherwise "riskier" businesses, a landlord often requires a personal guaranty from owners--they may need to review your financials to see if that is feasible or helpful.

3) If you are drawing excessive income from the business, that suggests that the business is not a good risk--you may be pulling out more money that it can support. Conversely, if the business can't afford to pay you a liveable salary, that suggests financial weakness.

Those are just a few reasons--many more exist.