Certain tenants can be a disruption when you try to sell a property and the misinformation that exists around this issue makes it an area of uncertainty for some people. It is certainly a tough process if you approach it in the wrong way so you firstly have to check over the tenancy agreement that you have in place.
Sitting tenants are those who are renting without a tenancy agreement and it is worth noting that it is difficult to avoid losing money on the sale of your property if it has tenants. The least problematic way to go about it is to give them notice of at least a month and to keep them informed throughout the process, even offering them first option on buying the place from you.
You may however be required to issue them with a Section 21 notice if you have any trouble in getting them to vacate the property. If you can avoid needing to evict them in this way then it is worth it but two months after issuing the notice you will be eligible to apply for a court order to remove them.