This is a loaded question as most people quite rightly don’t want to pay out money if they don’t have to. Firstly I would like to say that you are in the fitters hands, it is down to their discretion as they have to sign the appliance as safe. But a rule of thumb is that if a chimney is over 12”x12” it should be lined as it will slow the exit of gases causing chimney deposits and a risk of soot falls. Also if the chimney is in poor order, which is usually established by smoke testing the chimney, it should be lined. However stoves can be connected to clay liners if they are in good order.