Squatter housing, a sure tip-off to deep problems in national housing policy, is becoming a persistent sign of the times on both sides of the Atlantic.

This winter and spring several dozen families moved into abandoned houses in Philadelphia, Boston, Pittsburgh, Atlanta, Detroit, St. Louis and Tulsa. The activist organization behind the U.S. movement--ACORN, or Association of Community Organizations for Reform Now--promises that squatting will sweep into more cities unless the Reagan administration reacts with a broad new housing policy.

To gauge the full potential of illegal house possession, though, you have to visit Europe. Squatting began in earnest in the Netherlands, Great Britain and Germany in the mid-1970s. Today it has reached a peak in Berlin, where some 3,000 to 4,000 squatters now hold 138 apartment houses, braving official disapproval and occasional police-led evictions.

Squatters have learned to take security precautions, as I discovered in visiting an occupied tenement building in Berlin's aging Schoeneberg section. You first find the address on Bulowstrasse, then go to the rear and pass through the second courtyard, then find an entranceway, knock loudly, and wait until one of the occupants tosses a key down to you from the third floor.

Inside the decrepit 1890s walk-up building, one meets a dozen or so young squatters gathered around a big eating-socializing table in their community room. This tenement was "occupied" in January 1981; now it has 70 residents, aged 18 to 38. About half are students, the others are in every field from nursing to social work to carpentry.

Most of the interior is still incredibly shabby. But there are islands of renovation--whole rooms or corridors with refurbished and painted walls, new windows, new wiring and plumbing, and some handsome, naturally refinished plank-wood floors--the result of squatter investment, but even more squatter sweat equity.

Pastor Klaus Duntze, a Protestant clergyman who works with youth and neighborhood protest movements in West Berlin, told me that each squatter-occupied tenement has its own social profile. There are houses occupied by ex-convicts, by single retirees, by Turkish guest workers, and one by women only (it's nicknamed "the witch house" in Berlin squatter circles).

A form of communal living is under way at Bulowstrasse. The small one-bedroom apartments that lined the hallways are being modified by removing the old kitchens and bathrooms, giving each person a spacious single bedroom. Then, roomy community kitchens and living-dining rooms are installed at corridor ends, plus a large communal bathroom.

How can squatters justify illegally seizing other people's property? In Berlin, their answer is that basic housing--unlike a car, for instance--is such a fundamental need that it shouldn't be an object for financial speculation. They claim the city has 7,000 empty apartment buildings, while 60,000 people are registered as seeking housing.

In West Germany there's a shortage of one million housing units, while 700,000 apartments are unoccupied. And why? The squatters--and many outside observers--say it's because of the ineptness of government housing officials, plus tax laws that make it advantageous for owners to let their buildings run down until they're empty and can be converted to luxury units or demolished for commercial construction.

The same economic and government bungling has contributed to the United States' big reservoir of unoccupied buildings. In Philadelphia, there are 30,000 abandoned houses, many of them sturdy enough to justify renovation. The situation promises to get a lot worse. Rutgers University's George Sternlieb estimates demolition and abandonment is forcing an average yearly loss of 585,000 U.S. apartment units.

Now, high interest rates are paralyzing housing rehabilitation in U.S. cities, just as they have immobilized the new housing industry. Private owners who might have undertaken substantial renovations, of single houses or apartment buildings, either can't afford loans or don't see a payback down the road. Like deferred maintenance of roads, bridges and water systems, the negative effects may be small initially--but amount to a cumulative disaster over time.

Oddly enough, housing squatters may be doing a big favor for Europe and North America. Though their actions are technically illegal, they are occupying only long-deserted buildings. In a colorful, compelling way they underscore the blindness of governments that fail--by defective national economic policies and by sluggish local housing bureaucracies--to provide affordable housing and protect the treasure that a nation's old housing stock represents.