Christopher McCagg visits the Darien YMCA at least once a week. Not to run on the treadmill, swim in the pool or drop off kids.

"We have weekly construction meetings," said McCagg, the principal in charge and lead designer of the building's renovation. McCagg works for James G. Rogers Architects in South Norwalk.

"When I come, I'm looking for a lot of things," he said. "I'm there to make sure the design is being executed the way it was envisioned and to address and resolve any issues that come about as a collision between design and construction."

The last phase of construction under way is the main lobby and Holly Pond Nursery School. The project began in November and is expected to finish this September.

Upon a visit to the nursery school site, McCagg realized a number of harmless trees were going to be cut down where a playground would go behind the building.

"It's a shame," he thought at the time. "Why are we taking that down?"

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McCagg quickly raised the issue to his firm, which created a new design, and the YMCA approved it.

"The outcome is, we save a little money, we're environmentally responsible because we save the trees, and we're good neighbors because we provide screening," he said.

Cost efficiency, environmental friendliness and a sense of community are themes of the YMCA renovation that were built into its design. Take the cogeneration plant, for instance, a new gas-fired electric generator.

"As a result of the generator producing electricity, that unit gives off heat, and the heat is then recaptured and used to heat domestic hot water and the pool," McCagg said. "And the electricity is actually fed back into the grid. They're not producing electricity on site for themselves like your emergency generator would, but it's offsetting their energy costs back to the grid, while heating their domestic hot water in the pool very efficiently. It's basically free heat."

The heat from the generator also provides radiant floor heating in the nursery school.

"(It) is the best environment for children and for how to heat a space in general, given that it's a nursery school, and small children are always on the floor," he said.

It took nearly five years for the master plan to come together before the town approved it.

"Originally a more modern design was submitted to the town and was rejected," he said. "On short notice, we were asked to come back with a redesign, something more neighbor-friendly and contextually appropriate to the surrounding residential community."

First came construction on the gymnastics wing. The 10,000 square-foot room consists of four walls and a ceiling made of wood trusses that contribute to the building's residential quality. Construction on this facility took place March 2011 through November.

"Probably the biggest hurtle and the most complicated of this project is the logistics," McCagg said. "While this was being built, the old building was still up (and) the school was in operation. But we needed to continue to operate the school while we were building the new school. The most cost-effective option from all perspectives was to actually construct a temporary school inside the gymnastics space."

The gymnastics facility currently holds the temporary circulation desk, administrative offices, and the nursery school. It is made of collapsible interior walls, rugs, ceilings, air ducts, and a temporary wall dividing the room into two spaces.

"What will happen is, when we finish the new school, the children will move back into that, we'll finish off this space and move into it," he said. "We have to do some painting and some touch-up work; then we bring in all the gymnastics equipment and everything will settle back into its place."

Parents will soon drop off their kids in the driveway loop at the main entrance of the new nursery school. Visitors will enter the building underneath a pergola.

Once inside, the entrance to the corridor of the old building will be on the left, access to the nursery and administrative offices will be on the right, and a large, floor-to-ceiling window will look out on Holly Pond, while news and events play across TV screens.

"It's going to be a wonderful, active, energetic space," McCagg said.

Visitors may also notice something familiar.

"In making this facility unique to the community and adding a layer of warmth and comfort, we decided to go with the wood trusses in the gymnastics facility," he said. "Then, once we got through this half, we said, `Wouldn't it be great to tie the two halves together and really create the visual connection to the two spaces.?' It adds so much character."

tmichael@bcnnew.com; 203-972-4407