What's happening to Watertown's tax base?

The question came up in last night's neighborhood meeting at the Atrium School as to what happens tax-wise if the Atrium School decides to exercise its option to buy the building at 69 Grove Street. The current arrangement is a 20-year lease with three five-year extensions at Atrium’s discretion and an option to buy/first refusal. The school wanted to purchase the building but it was too expensive. The answer of course is that, because The Atrium School is a non-profit, the building would become non-taxable. Since 69 Grove Street is currently assessed at $2,058,800 that would mean a loss to the town of about $42,000 each year.

So I was thinking about what other commercial sites in town have been converted to residential recently to see if we're losing our tax base. Now I may be completely wrong with the math here but it seems like most of the conversions represent, at face value, a net tax revenue gain.

Here's an example . The former Quincy Market Cold Storage on Pleasant Street was assessed at $7,975,000. So the town was collecting about $162,300 each year (at the $20.35/thousand commercial rate). To get that much from a residential development there has to be $14,530,000 in assessed value (at the $11.17/thousand residential rate). That's not difficult to achieve when you are building 375 units.

There are the Riverbank Lofts on Pleasant Street that were originally supposed to be high tech offices. That property, assessed at about $4.5 million will have 57 units that will need to be assessed on average $145,600 to bring in the same amount of revenue. I would guess they'll probably sell for at least twice that.

Another is the development on School Street where The Wadi used to be. And the Aggregate Industries site on Grove Street that might become a residential development.

So it seems like our tax base, though converting to residential, is actually growing. But then you have to consider whether each additional resident is a net tax gain or tax loss for the town. It costs the town a lot more to provide services to the residents of condominiums and townhouses than what it has to provide to a big warehouse. I'd be interested to see numbers on this.