That's what I think.
Rough figures:
eg 7 x 4 m swimming pool = 28 x €200 = 5600 x say 5% tax (varies by department) = €280 tax
10 x 20m extension = 200 x €800 = 160,000 x say 5% tax = €8000 tax
I haven't looked into it much but that's my first impression based on what I've read
nb €200 and €800 amounts are not exact but I recall that's roughly what they were unless you were in Ile de France