The short answer is “only the landlord, or someone else with landlord’s consent.” But, there is a little more to this answer for landlords with properties in Missouri.
General rule
The general rule for all rental units is that except as provided by law, the tenant must not make any repairs or alterations to the premises without the prior written consent of the landlord and the homeowners association, if applicable. Repairs and alterations include but are not limited to painting, wallpapering, demolition, carpentry, installation of fixtures, or any other changes to the premises. If a landlord does provide consent to repairs or alterations taken on by the tenant, those repairs or alterations must conform to a professional standard of quality. It’s also important for tenants to understand that any repairs or alterations they perform (or cause to be performed) will become the property of the landlord, and the tenant shall not be entitled to any compensation for the repairs or alterations.
Regarding keys and locks, the tenant can not alter or install any locks to the premises without the prior written approval of the landlord. The same is true for a security system. If the tenant does receive permission to install new locks or a security system a copy of the keys and instructions for disarming a security system must be provided to the landlord.
Repair and deduct
Missouri has a “repair and deduct” remedy for the tenant if there exists a condition on the premises which detrimentally affects the habitability, sanitation, or security of the premises, and the condition constitutes a violation of a local municipal housing or building code. Per Mo. Rev. Stat. § 441.234, if the reasonable cost to correct the condition is less than three hundred dollars, or one-half of the periodic rent, whichever is greater, and the amount does not exceed one month’s rent, then the tenant can notify the landlord of their intention to correct the condition at the landlord’s expense. This notification is important, because the landlord is afforded 14 days after being notified to correct the condition (or more promptly in the event of an emergency). If the landlord fails to correct the condition within that time period then the tenant can cause the work to be done in a workmanlike manner. The tenant then submits to the landlord an itemized statement, including receipts, for the cost of the work done. This amount can deduct the cost of the repairs, subject to the limits noted above, from the next periodic rent due.
There’s a little more work to be done on the part of the tenant, however, if the landlord disputes the necessity of the repair during that 14 day period. In this case, the tenant can not proceed with the repair work without a written certification from the local municipality or government entity that the condition requiring repair constitutes a violation of local municipal housing or building code. Once the tenant has the proper certification they can then cause the work to be done if the landlord fails to correct the condition within 14 days after the date of the certification or the date of the notice from the tenant, whichever is later. Of course, in cases of an emergency the tenant can accelerate that timetable.
These repair and deduct provisions are only available to the tenant if the tenant has lawfully resided on the premises for six consecutive months, has paid all rent and charges due during that time, and did not during that time receive any written notice from the landlord of any violation of any lease provision or house rule, which violation was not corrected.
Finally, the tenant is not allowed this repair and deduct option if the condition was caused by the deliberate or negligent act or omission of the tenant, a member of the tenant’s family, or other person on the premises with tenant’s consent. It’s important to note that the tenant may not deduct in total more than the amount of one month’s rent during any twelve-month period.
