A tenant should not make any improvements or repairs to a landlord’s property without the prior written consent of the landlord.
When the tenant moves out of a rented property, he may take with him his linens, kitchen utensils, chairs and piano, etc. However, if he has at his own expense replaced a windowpane or nailed back some loose boards, he cannot take with him the nails or the glass. The nails and the glass are now "fixtures": items which have been so attached to or associated with the property that in the eyes of the law they are considered part of the property and may not be removed by the tenant.
An exception to this rule "permits the tenant to remove articles and structures designed for the purposes of trade, domestic convenience, or ornament, even though they are so firmly attached to the realty that the tenant would not otherwise have the right to remove them, provided they can be removed without serious injury to the premises." (Maryland Law Encyclopedia, Fixtures, Section 5)
Despite the exception, tenants are not always protected. For example, a tenants may have asked a landlord if he could get rid of something such as an old refrigerator which was no longer working properly, in order to replace it with a new refrigerator purchased by the tenant. The landlord gave might permission orally, but deny it at the end of the tenancy and refuse to allow the tenant to take the refrigerator when he moves out.
The tenant should also be very careful about oral agreements for him to make repairs in lieu of rent or for reduced rent. The lease or other written agreement should specify exactly what repairs are to be made by the tenant, the time frame, if relevant, the quality of the supplies and who is to pay for them, and the terms of the tenancy when the work has been completed.
An example is that of a tenant who had a furnace problem on a weekend, had it fixed and wanted to deduct the cost from the next month's rent. If the tenant refuses to pay the rent, the landlord may take the tenant to court, where the landlord will probably win. The tenant should have notified the landlord about the furnace and asked him to fix it or give the tenant permission to do so before making the repairs.