For years Carrboro has required developers to make 15 percent of the units they build affordable to buyers who make 80 percent of this area's median income.

By that standard, The Butler, the 57-unit condominium project the aldermen approved last week, should include nine affordable units.

Instead, it will have only five.

In place of the other four, the developer, Urban Ventures LLC, will pay up to $100,000 per unit into the town's affordable housing fund.

The arrangement marks the first time the town has agreed to a payment-in-lieu in a situation like this, allowing a developer to pony up some cash for affordable housing in exchange for building fewer than the required number of affordable units.

That comes as welcome news to local affordable housing organizations such as the Orange Community Land Trust and Habitat for Humanity.

Leaders of those groups say money is often more useful than the kind of affordable units many developers build in order to meet the requirements. Often those units are tiny -- very tiny. They're affordable, yes, but they're really suitable only for a very narrow demographic, primarily singles without children.

Cash, on the other hand, is always in short supply and provides affordable housing groups the ability to do all sorts of things: buy land, build homes suitable for families, maintain existing dwellings, fund programs, hire needed staff and so on. Additional money for those functions may be more valuable than a scattering of tiny units in new developments.

Oversight and planning are essential, of course. Money paid into the affordable housing pot should be used only for affordable housing purposes. But assuming the process is carefully administered, the payment-in-lieu option is a useful device.

The people whose daily work is affordable housing are in a position to know what kind of resources are most useful. Money gives them the flexibility to direct their efforts where they can be put to the best use.

Both the Carrboro and Chapel Hill governments have long been proactive in working to find ways to leverage more affordable housing -- not easy in a market that encourages the building of upscale, expensive homes.

The 15 percent standard is one of the tools in that effort. Payment-in-lieu is another, related one. Employed wisely it's a valuable asset.