The gleaming new solar panels on Trolley Square send a clear message to passers-by that this new affordable mid-rise project is a green building designed to use less energy than conventional construction.

However, Homeowner’s Rehab, Inc. (HRI), the nonprofit that developed Trolley Square, only put solar panels on the building as an afterthought, after it had already committed to make the 40- unit building in Cambridge, Mass., more energy efficient in a host of less flashy ways. “I think of the solar panels as the icing on the cake,” said Jane Jones, a senior project manager with HRI.

Solar panels and roofs planted with grass, the technologies most strongly associated with environmentally conscious design, are often the last things green builders include in their projects. “The first thing you have to do is build a good building,” said Andy Padian, senior housing specialist and green building expert with Steven Winter Associates, Inc., based in New York City. For Padian, green construction starts with rooms in which air does not leak in through chinks in the walls and floors, and which receive enough fresh air through good ventilation. That’s hardly a cutting-edge goal, but few conventional buildings seem capable of achieving it.

Green design is gaining popularity in affordable housing circles partly because of state incentives. Most states now give affordable housing developments that include green features an advantage in the competition for low-income housing tax credits, according to an analysis by U.S. Global Green. Seventeen states favor affordable projects whose features combine Smart Growth, energy efficiency, resource conservation, and health protection. Massachusetts has the strongest incentives, followed closely by Georgia and California.

One affordable housing nonprofit group has also helped stimulate the production of green affordable housing developments. Enterprise Community Partners, Inc., has invested more than $425 million to create more sustainable homes through its Green Communities initiative. In 2008, Enterprise estimates that one out of every four tax credit investments it makes will help finance green developments.

Low-cost ideas with big benefits

Often, the most effective green innovations are the most mundane. For example, dual-flush toilets, which allow water usage to be adjusted, are so effective at saving water that they pay for themselves within months, said Ross Speer, a principal with Mostue & Associates, the architects for Trolley Square. In all, energy-efficiency measures added just $300,000 to the project’s $15 million development cost.

Even at developments known for dramatic green features, the basics can make a big contribution to an improved environmental footprint.

One of the most comprehensive examples of green design is Solara. The 56-unit affordable housing development in Poway, Calif., is the first apartment complex in California to be powered entirely by solar energy. Its $1.1 million solar panel installation was almost entirely paid for with rebates and tax credits.

Community HousingWorks (CHW), the San Diego-based nonprofit that developed the community, also focused on green building basics by choosing a site near mass transit and services and working to save water and use recycled or renewable construction materials. Hot water is supplied through a tankless system that also acts as the heating system for the apartments.

CHW also incorporated low-flow devices and other water-saving measures. It improved indoor air quality by keeping heating, ventilation, and air conditioning ducts closed during construction. And the developer planned for long-term sustainability by developing a green education program for tenants and having them sign leases that include green provisions.

Winning over contractors

One obstacle to green design: Contractors often charge extra to try unfamiliar techniques. Affordable housing developer Pennrose Properties had to pay a 50 percent premium to get its framing contractor to try stacked framing at the Eastampton Town Center project in Eastampton, N.J.

The technique uses 2-by-6-inch boards to frame a building instead of the usual 2-by-4s. Because the wider boards can support more weight, less lumber is needed.

The New Jersey Department of Community Affairs paid for the stacked framing at Eastampton through a pilot program to support green building techniques. The contractor liked the technique so much it now uses stacked framing at no extra cost on all its work for Pennrose.

To help implement green construction techniques, a growing number of developers recommend hiring a green building expert called a “commissioning engineer” to look over the project plans and then inspect the site to make sure the green features have been properly installed. HRI paid a commissioning engineer $20,000 to visit Trolley Square.

Covering the costs

Developers like Jones of HRI typically don’t include green improvements unless they can generate enough savings to cover their costs within 10 years.

Some of the green features with the highest sex appeal flunk this test. For example, the solar panels at Trolley Square can provide enough power to save the building roughly $600 a month in electricity. At that rate, it would take the $500,000 in panels 24 years to pay for themselves. However, developers such as HRI are still willing to incorporate the features when they can find funding to offset their costs. At Trolley Square, a grant covered the full cost of the solar panels.

It’s also nice to have something that passers-by can identify as a green feature, said Jones. Perhaps that’s why the solar panels are bolted to the west side of the building facing the street instead of tilted on the roof, where they might collect more energy.