Cancelling, and what the notice decides
What is kept when a booking is called off depends on the notice given rather than the reason, and nothing beyond the deposit is ever claimed.
Cancelling never makes you liable to the house for anything beyond the deposit. What may be kept is the share of the fee already earned when your message lands, and the only thing that sets that share is how much notice you gave.
The scale
More than twenty-four hours before the booked time: nothing is kept and the whole deposit comes back. Inside the final twenty-four hours, with the therapist not yet on her way: half the deposit is kept as fee already earned and half is repaid. Once she has set out: the deposit is kept entire.
Two things that reduce even the last step
If the hour you released is taken by another client, the deposit is repaid in full, because the time was not lost. And if you can show the loss to the house was smaller than the part it kept, it keeps only that smaller amount.
A booking with no deposit behind it
The returning client's same-day hour or two at a city address carries nothing in advance, so the scale has nothing to act on. Called off before she has left, that booking leaves nothing owed. Called off once she is already crossing the city, it costs the part of the fee her journey represents, and nothing past that is asked for.
Moving instead of cancelling
On a trip built on two flights, a move is usually what is wanted. One change of date is free with more than a day's notice and a new date inside fourteen days, and the deposit travels across whole. The rest, including what happens when something genuinely outside either side's control intervenes, is on the deposit page.