9:12 AM PT -- Sofia Vergara's attorney, Marty Singer, tells TMZ it was actually Sofia who first sent a demand letter to RCH on October 4 ... claiming the company was responsible for $5 million in damages for overcharging Sofia, doing substandard work and delaying the project.

Singer says RCH never disputed the claims, but has now filed a lawsuit of their own ... Singer says he's confident Sofia will prevail and receive significant payment.

Sofia Vergara skipped out on paying a contractor more than $1.7 million for a huge renovation of her luxurious home in Beverly Hills ... or so the contractor claims in a new lawsuit.

According to the suit, obtained by TMZ, the actress hired Reside Custom Homes in April 2022 to perform construction work on her $26 million mansion located in the Beverly Park neighborhood.

RCH claims Vergara agreed to pay for the work plus the contractor's fee, but claims she never followed through once the work was done.

RCH says the project was substantially completed by December 2022, but then Vergara allegedly asked for significant extra work beyond the scope of the agreement, which ended up extending the job until March 2023 ... when Sofia finally moved back into the home.

According to RCH, Vergara asked the contractor to install clay plaster in 3 powder rooms, an office, the dining room walls, the cabana bathroom and the guest house living room. She also wanted the company to remove and replace a flight of stairs.

RCH says it fronted the money for the additional work in order to finish the improvements and claims Vergara promised to pay the bill ... even allegedly telling RCH she'd approved one of the invoices for nearly $900K and "already" gave directions to her money manager to pay the bill.

But RCH says Sofia never followed through and now owes the company a whopping balance of $1,700,492.64.

We've reached out to Vergara's rep ... so far no word back.