YOU WOULDN'T add a room onto your house without knowing how it might affect your taxes or utility bills. So why are local governments expected to approve residential developments without knowing what impact they'll have on schools, emergency services and other cost drivers?

That question is key right now in Anne Arundel County, where plans for a new sewer line recently showed up without any assessment of its fiscal impact. The County Council understandably extended icy greetings to the project.

Howard County officials went even further last month when they voted to require fiscal-impact studies for all mixed-use developments, like the 1,168-home project planned along Route 216 in Fulton.

Community activists have long held that residential development doesn't pay for itself and that most new homes are a liability to government coffers.

Their extreme position ignores the fact that new business follows housing growth.

But the activists are right that government officials ought to demand more details about developments before houses spring up and services become necessary, no matter their cost.

There is one caveat, though: The numbers in fiscal impact statements can sometimes be as useful in the long term as today's closing stock market prices. Estimating development's impact isn't an exact science. It's a resource and not the last word on growth.

Politicians should require more information about how development will affect county finances, but they shouldn't twist the projections into support for anti-development stances that may not be the best alternative.