The most common and most likely answer to this crossword clue is the 14 letter word CONSERVATORIES
I've seen this crossword clue in The Times
C
O
N
S
E
R
V
A
T
O
R
I
E
S
Buddy explains!

The straight part
We know that "rooms on ground floor" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rooms on ground floor = conservatories.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does CONSERVATORIES mean?
CONSERVATORIES noun- That which preserves from injury.
- A storehouse.
- A large greenhouse or hothouse for the display of plants
- A glass-walled and -roofed room in a house
- A school of music or drama