Marilyn Monroe's L.A.-area home will not be listed for sale ... despite the residence being named a Historic-Cultural Monument, which upends the current owners' plans to demo the pad.

Sources close to the homeowners tell TMZ ... Brinah Milstein and Roy Bank have no plans to sell the Brentwood home -- where the Hollywood legend died of a barbiturate overdose in August 1962.

Rather, we're told the married couple will continue to fight this week's L.A. City Council unanimous vote to designate Marilyn's former house a landmark with cultural significance -- a tag that protects it from being torn down.

We're told Brinah and Roy feel violated by the ruling and don't want to be forced to sell it ... especially because the property was meant to be an expansion of their family home, which sits right next door.

Not only that, we're told the house -- which the pair bought for $8.35 million last year -- is in a state of total disrepair, which is why they don't reside there.

Our sources say there are major roof issues causing leaks inside the house, but Brinah and Roy have no plans to fix any of that now ... as the new designation makes it very complicated to do anything to the house.

Essentially, if they can't demolish the home, the property's useless to them -- and that's why they're continuing with a lawsuit fighting the historic designation.

We're told the couple's faced threats over their lawsuit, and they constantly have to explain to fans the home isn't a public place ... and probably never will be.

The Marilyn Remembered Fan Club certainly feels differently, as a spokesperson tells TMZ ... the club fully supports the designation, because Marilyn's impact on the film industry is worth commemorating.

Their bottom line is ... protecting the home is protecting Marilyn's legacy, so they're not backing down -- but neither are the homeowners.