The tax will be 20% of the property’s “valeur locative cadastrale” (a theoretical value). You can find this value at www.cadastre.gouv.fr. So if a property has a rental value of 3000 euros then the annual tax would be 20% of 3000 euros – i.e. 600 euros per year.
First, this does not apply to expats who are now living in France permanently. Although there are separate proposed changes to their wealth tax.
Second, if you ensure that your home(s) is available for rent for the whole year via a local letting agent then there will be no charge.
At this stage there are no indications how the tax will be applied if the property is not available for the full year.
The new tax is something Americans thinking of purchasing property in France will need to consider, however according to the French Property blog:
"It is yet to see if the new 20% tax will be introduced, as many believe it can be legally challenged under European Law as it discriminates against foreign home owners. The Spanish Government were successful challenged a couple of years ago on similar grounds, when they charged non Spanish residents higher capital gains tax on the disposal of property. However, both Italy and Spain have a similar tax regime for non-resident property owners."