For the area of a (rental) home, I would assume that 35 m² means 35 m² of indoor area, excluding only the garden (if applicable). That means, if we rent a German apartment that is 35 m², it will be the same size as where we live now in England. However, from this (previous revision, now edited) answer by Martin Bonner, it appears it may not be that simple:
It is quite common in Germany for not all space within an apartment to count - toilets/bathrooms may not count at all, kitchens/balconies count half, and living rooms/bedrooms are fully counted. (...) if you got the 100sqm from the property details (rather than your husband getting out a tape measure), they will already have done that pro-rating.
Is this a general rule? For example, does that mean that if an apartment somewhere in southern Hessen is advertised as 35 m² on Immobilienscout or another real estate website, it is actually even larger than 35 m²?