In the Netherlands, you are allowed to install an air conditioner in a rental property, but there are rules and conditions to consider. The main points are:

Landlord's consent

Mandatory: To install a fixed air conditioner (such as a split unit with an outdoor unit), you should always seek written permission from your landlord first. This installation may affect the house structure or appearance, which often may not be altered without permission.

Landlord conditions

Appointments: The landlord may set specific conditions for the installation, such as the location of the outdoor unit, noise levels, or aesthetic considerations.
Return to original condition: At the end of the lease, the landlord may require you to return the property to its original condition. This means you must remove the air conditioner and repair any modifications or damage.

Legal and construction considerations

Permits: In some cases, depending on the location and type of property, a permit from the municipality may be required to install an outdoor unit.
Building requirements: The installation must comply with building requirements and must not cause nuisance or damage to the building or its surroundings.

Responsibility for maintenance and damage
Maintenance: If the air conditioner was installed by you, you are responsible for its maintenance. This includes repairing any damage caused by the installation or use of the air conditioner.

Costs: All costs for installation, maintenance, and possible removal are usually for the tenant, unless otherwise agreed.

Special situations
Owners' Association (VvE): If the property is part of an apartment complex with an owners' association, the rules of the association must also be followed. This may impose additional restrictions on the installation of an outdoor unit.

Summary
In principle, you can install an air conditioner in a rental property, but you should always ask the landlord's permission for a fixed installation. It is important to make proper arrangements about the conditions, maintenance, and any return to its original state when the tenancy ends.