Mortgage interest

Dear Sir/Madam, I read in one of your articles that there are three important reasons not to include your home loan, for example when selling (https://dexxter.be/drie-redenen-kantoor-huis-beter-niet-inbrengen-als-kost/#:~:text=The%20parents%20are%20going%20to%20rent%20it%20out%20for%20professional%20activities). Does the above article refer to writing off the purchase value (e.g. the space is 10% of a house from £300,000 and therefore worth £30,000), or does it refer to including the interest on your mortgage loan? In other words, is it disadvantageous to include the interest on your loan, or are there no disadvantages associated with this?

Thanks in advance!