NEW YORK — In a gentrifying San Francisco neighborhood, a couple exit their cab and head toward an apartment, rolling suitcases behind them. Unbeknownst to them, a private investigator, Michael Joffe, sits in his parked car across the street, discreetly snapping pictures.

This is not a divorce case or an international spy caper — nothing that salacious or mysterious. It is instead an episode that provides a window into how bitter the feud between struggling tenants and home-sharing websites like Airbnb Inc. has become. Joffe works for a tenant lawyer who in turn represents a family that was evicted from their apartment — the one that the couple was entering that day.

The goal of the stakeout was to document that the landlord was violating city ordinances that limit the use of private homes for short-term rentals. It’s a lucrative business nowadays in San Francisco, the city that’s come to represent America’s shortage of affordable housing.

“Unfortunately, or fortunately, depending on how you want to look at it, it’s a decent living in San Francisco right now, being an investigator doing these kind of jobs, because here are so many of them,’’ Joffe, 48, said.

Airbnb disputes that home-sharing has significantly reduced the supply of housing for the poor and moderate-income, pointing the finger instead at rising demand and restrictions on building new units.

But in recent weeks it has taken steps to comfort alarmed officials, a sign perhaps that aggressive steps by tenants are helping to sway the debate. Starting in November, for example, Airbnb instituted a “one host, one home’’ policy in San Francisco and New York as a way to knock out investors who may be collecting apartments to market on the Web for short stays.

“We strongly oppose illegal hotels and bad actors who remove housing from the market,’’ said company spokesman Nick Papas. “We’ve removed thousands of listings from our platform that aren’t right for our community.’’

Still, with municipal governments lacking the staffing to enforce housing ordinances, there’s no shortage of work for private eyes like Joffe. When he’s not taking pictures of people coming and going, he sometimes poses as a prospective tenant to see if landlords allow short-term rentals.

One case he looked into involved Brian Grzybowski, who claimed in court papers that he and his wife were forced to leave their $2,950-a-month Potrero Hill apartment in 2015 after the landlord falsely claimed the apartment was needed as a permanent residence for a family member.

The unit soon popped up on Airbnb, Craigslist, FlipKey, Zeus Living, and Tripping.com, according to a complaint filed in California Superior Court. Now the Grzybowskis are paying $5,500 a month to live a block away. It’s against San Francisco law to evict someone for the purpose of leasing the apartment short-term.

Joffe’s job involved inspecting the unit, in the presence of attorneys from both sides, to determine that it was set up for short stays. One finding: It had small soaps and shampoo bottles, as a hotel room would.

The suit was settled in a way that didn’t require an admission of liability, said John Brydon, whose law firm represented the landlord.

In many cities, people on the wrong side of the median income are feeling the pinch.

In New York, a June study by two nonprofits that advocate for affordable housing found that the top 20 neighborhoods for Airbnb listings in Manhattan and Brooklyn had average rent increases almost twice those found in the city as a whole between 2011 and 2015. In the Soho/Greenwich Village area of Manhattan, for example, a short-term rental can bring in close to $10,000 a year.

A study in San Francisco found that neighborhoods with the highest number of evictions in a one-year period also had the highest number of commercial hosts on Airbnb.

The study noted the boost the city got from tourists who could not afford its hotels. And for many people, the income from renting couches or extra rooms allows them to afford to remain in San Francisco.

Still, Airbnb cooperated with limits on home-sharing in London and Amsterdam and dropped a suit challenging fines against people who post apartments in New York that were illegally converted to short-term rentals. It also praised rules in New Orleans that limit rentals to 90 days a year in non-owner-occupied homes.