In the latter case, the cost of journeys to other workplaces is eligible for tax relief. There is also what economists describe as an opportunity cost, in that the space you are using at home for work can no longer be used for other purposes. It's sensible to install a separate work telephone line for home working, and most employers automatically meet this cost for their employees. Some, but by no means all, employers also contribute towards the other costs incurred, typically through a regular lump-sum payment. However, this sort of payment can be taxable as a perk, if it cannot be justified as legitimate.
Sensible employers negotiate any home expense payments with their tax office first.
For the self-employed, the test is less rigorous. Expenses incurred 'wholly and exclusively' for work can be set against tax. According to the Council of Mortgage Lenders, you should let your mortgage company know that you are working from home. In practice, most people fail to do so, perhaps concerned that their mortgage will rapidly be changed to a higher-rate commercial loan. The CML says that, whilst conceivable, this is unlikely.