Bath and North East Somerset Council said the £45 million building had been plagued by a raft of defects which had seen it incur “significant costs” before and after the 2006 opening.
It said it now wants £21,366,318 from the construction firm, plus costs.
The legal claim comes in response to one filed over the summer by Carillion, which bought the spa’s actual main contractor Mowlem two years ago.
Carillion served a writ against the council in August seeking £2.4 million in compensation, although the council said they had originally indicated they would be suing for around £10 million.
A council statement said: “As a result of extensive clarifying discussions and negotiations with Carillion prior to the issue of the writ, the amount of the Carillion claim has been significantly reduced.
“The council has consistently maintained that it has no liability to Carillion but rather that it has significant claims against them and/or other parties in connection with design and/or workmanship and/or supervision related to the Bath Spa project.”
It is understood the building has had a series of problems, from leaking pools to steam-room doors that warped with the heat.
It was also beset by a series of delays, the result of a range of problems from nesting birds to an archaeological dig that ran late.
The council, which claims responsibility is down to design and workmanship failings, has also served a notice on architects Nicholas Grimshaw & Partners which will involve them as a party to the litigation.
It said: “Carillion has made allegations as to design failures in relation to the Spa against NGP and the Council will be passing on these allegations and/or otherwise holding NGP responsible for design and/or supervision failures in relation to the project.”
The statement said the council had “sought to resolve matters constructively without recourse to the courts” and only filed the claim after Carillion served its own writ.
Carillion refused to comment on the dispute.