My landlord is going to let a new tenant put a patio in a common area in front of our building -- and right in front of my grooming shop. This will make it practically impossible for passersby to see my shop from the street, and I will lose an important source of new customers. Can I break my lease?
It's amazing the disregard some people have for small business needs. But there is a chance that you can escape from your lease.
First, check the lease itself. Chances are slim, but the lease just might require your landlord to maintain open access to your space and to permit unimpeded visibility from the street. If that doesn't work, check what your lease says about the common areas -- if it specifies that they are to remain open for use by all tenants, maybe the patio use violates that provision.
Finally, look for a "signage" clause. If the landlord has agreed to allow you to place signs on the building or in your windows, and if the new patio will make those signs ineffective, you may be able to argue that by allowing the patio, the landlord will indirectly break, or breach, the clause.
If any of these arguments seems to fit, look for a "disputes" clause, which will specify what happens when one of you claims the other is not living up to the lease. Hopefully, it will point you towards mediation, not a lawyer's office.
If there's nothing specific in the lease that applies to your situation, you'll have to fall back on an "implied contract." In other words, you would argue that, taking the lease as a whole, it's implied that the landlord won't do something that drastically changes the building and common areas and diminishes the value of your space. Whether this will work depends on the specific terms of your lease -- you'll probably have to review this with your lawyer.
But before you begin packing up, have a talk with your landlord. (In fact, if your lease has a dispute resolution clause, you may be obligated to try to work things out before deliberately breaking the lease.) You may find that the landlord is willing to negotiate an early termination of your lease. In the long run, it will be better than risking a lawsuit, which will not be short or inexpensive -- even if you ultimately prevail.
If the patio hasn't gone up yet, make sure you take before and after pictures. They can help enormously if you end up having to prove your case in court.