Does cost basis of house shift upon death of a spouse to fair market value of home on date of death?

Home purchased in 1954 for $15000.  Spouse died in 1992.  Surviving spouse sold house for $685,000 in 2017.  Does adjusted cost basis of house remain at $15,000 plus improvements, or does it step up to fair market value of home on date of spouse death.  House is in CT.