A quote reflects what is known at the outset. If something unexpected surfaces, such as a title defect, an unregistered extension or a leasehold with an awkward management structure, genuine extra work follows and there may be a further fee. What should never happen is discovering it on the final bill. Any additional charge should be explained when it arises and set out in your terms of business. If a quote looked unusually cheap at the start, this is often where the difference reappears.