See this related thread. Apparently, it’s not possible to change the step size for setting the target temperature for thermostats. I can confirm this is true for Plugwise thermostats. Those seem to default to 0.1ºC. However, I have the impression that this is a generic problem for thermostats.
While I can imagine scenarios where setting the target temperature in 0.1ºC steps is useful, I think in the majority of cases, a 0.5ºC step size is way more useful. It is accurate enough, while it is also possible to do a larger change in just a few button clicks. Also, the physical + and − buttons on the Plugwise thermostats do have a 0.5ºC step size. I think it’s more intuitive for end users if the step sizes match.
There is a work around possible, by adding the following to the configuration.yaml:
homeassistant:
customize:
climate.thermostat_entity:
target_temp_step: 0.5
However, this is apparently not the preferred way to do this. My documentation PR where I wanted to add this work around to the docs, got closed with this remark:
This should not be used for that, nor do we recommend the use of customize in general, as this overrides/injects things directly into the state machine (and may cause problems of its own).
For that reason, I’m going to friendly decline this addition to the documentation.
Which is fair enough, but also another reason to implement a proper solution.