Many leases and rental agreements forbid tenants from making any “repairs, alterations, or improvements.” Even if you don’t have such a clause, check with the landlord before you make any improvements such as a built-in closet system) or repairs, especially major repairs that have the potential to get complicated or expensive, or have dire consequences if things go wrong. See the Nolo article How to Get Your Landlord to Make Major Repairs for advice.
Even if the repair is minor, it’s a good idea to get the landlord’s permission. You can use this Sample Letter Requesting Permission to Do a Minor Repair as a model in preparing your own. This letter is written in a way that tells the landlord you plan to make the minor repair unless you hear to the contrary within a reasonable amount of time. In this sample, the tenant has explained that she knows the repair problem, has experience with similar repairs, and a plan for making the repair—points that are designed to inspire the landlord’s confidence.