“While walking by 12 Chestnut I noticed recent construction.

Dave Long, the contractor, told me the building was going

from condos to a single family. I wondered if this was

unique, the reversing back.”

This is an email a reader sent in early September. It got me wondering. Since the 1970s multi-unit buildings in downtown Boston seem to have undergone all kinds of changes, usually resulting in fewer units. Upper Pinckney Street on Beacon Hill was particularly affected early in my observations when many families renovated homes that had been divided into apartments.

I’ve participated in the change. My own building used to be four units and is now two. Next door the building went from four to three. Next door to it the building went from four apartments to three, and now it is a single-family. And these are tenements. So buildings that began life as single-family houses must have seen even more changes.

But I had only anecdotal evidence. Tom Clemens, who has been the Beacon Hill Civic Association’s zoning chair for many years, said he had only anecdotal evidence too, since increasing the number of units, not reducing them, is the matter that comes before the zoning committee.

Mark Melnik, the Boston Redevelopment Authority’s deputy director of research, had the facts. He analyzed the numbers coming from the assessing department.

In most of Boston, there is no such trend, he said. Three deckers apparently stay three deckers. But in downtown Boston and Charlestown it has been a different story. Melnik looked at statistics from the year 2000 until 2008 and called it a phenomenon. Both apartments and condominium buildings are being converted.

Look at Charlestown. Forty-four two-family homes were converted into single-family homes in the eight-year period. That’s almost 10 percent, since there were 481 two-family homes in the year 2000.

In the North End and other parts of Ward 3, the story was similar. Two-family homes were the most likely to be converted. Again the percentage that had undergone conversion to single-family housing was almost 10 percent.

In the Back Bay and on Beacon Hill, conversions were even greater. Twenty-two percent or 31 two-family homes were converted to single-family use over the eight-year period compared to 404 in the whole city. Almost three and a half percent of the buildings housing three families or more were converted.

Why?

Downtown has become the place to live. The city’s downtown residents are wealthier, so they can afford larger living spaces. Downtown Boston and Charlestown are filled with beautiful architecture, and their occupants can walk to work. Who wouldn’t want to live here? And everyone needs more space.

This phenomenon means that in the downtown neighborhoods and Charlestown there is less density than historically has been the case. But new units have been added in these neighborhoods. Institutional buildings like Northeast Institute, a trade school on Phillips Street on Beacon Hill, have been repurposed into upmarket condominiums. So it isn’t a clean break with density.

The construction in the house the reader is watching has been needed for a long time. That’s typical, especially when the units have been rentals owned by a neglectful absentee landlord. Electrical systems need a complete overhaul. Insulation is required by the city. The plumbing is on its last legs. Extra kitchens must be removed and a new family kitchen installed. Awkward walls built to create multi-family units don’t match the originals and have to go. Once you’re tearing out all the systems and inappropriate construction, it is a small step to redoing the whole thing.

I happen to know the people who are converting 12 Chestnut back to a single-family house. It’s a family of four. The wife is in the finance industry, the husband in publishing. Their two sons have been involved in Hill House activities for several years. They live in the Back Bay now, and are eager to move into their new home, which should happen sometime this spring.

The husband bought The Beacon Hill Times from me three years ago. Now he’s my boss.

Give them a big welcome when they move in.