@tanguatlay,
If you read out 0.5 ha, you would read it as
"zero point five hectares " (or "point five hectares" - the zero is implied)
It sounds wrong to say 'zero point five hectare', with no s. Try it.
If you add the 'of a', or just 'a' with the of implied, then the 'a' is 1, and hence you are talking about a singular noun, and thus have no s
"point five of a hectare"
"a hectare" is singular.
You can also say it as
" a half hectare"
Here, 'of a' is implied, the full phrase being "a half of a hectare". So again, it's singular.
With the phrase "point five hectares", there is no implied 'of a', hence the plural.
Note that 25.5 hectares would be pronounced as
"Twenty five point 5 hectares" or "Twenty five and a half hectares"
and never "twenty five and a half of a hectare", which is just plain wrong. The 'of a' is only for fractions in their own right.
(confused? Welcome to the English language! ;-) )
T.