A proposal to develop an office center at the downtown landmark National Hall won praise from Planning and Zoning Commission members Thursday night.

That support was tempered, however, by zoning officials' concerns about the project's potential impact on parking and traffic around the site on the west bank of the Saugatuck River.

Greenfield Partners, a Norwalk-based commercial real estate firm, is seeking to relocate to National Hall -- an icon that has been vacant since its last occupant, the Inn at National Hall, closed in March 2011. Greenfield, which owns the building, wants to convert the property in the town's Historic Design District into an approximately 18,400-square-foot office center on three floors. Greenfield's site plan also calls for approximately 1,100-square-feet of first-floor retail space, an approximately 1,600-square-foot glass-facade addition to National Hall and a new public plaza overlooking the Saugatuck River.

"I think this application comprises the best opportunity to incorporate the HDD as a vibrant, active part of an expanded downtown," Bill Fitzpatrick, Greenfield's land-use counsel, told the P&Z.

The revitalization of National Hall ties into an ambitious plan by Greenfield Partners to redevelop a cluster of buildings it owns on the west side of downtown Westport. The P&Z has received site plan applications for a new restaurant each at 6 Wilton Road and 12 Wilton Road. Greenfield also plans to eventually recruit restaurant and office tenants to move into the currently 20 Wilton Road building.

"We're very excited about the project," said P&Z Chairwoman Catherine Walsh. "We have a lot of concerns about the parking. You have to figure out how to address it. It's not just this building or that building; it's the whole site."

Greenfield employees would park at a Greenfield-owned parking structure on Wilton Road, which borders the National Hall complex to the west. The parking deck currently holds 97 vehicles, but Greenfield won P&Z approval in March to add a third level, which would increase the parking complex's capacity from 97 to 133 spaces.

While Greenfield's site plan supports the minimum number of parking spaces required by town zoning regulations to accommodate an office complex at National Hall, its lack of on-site parking spaces does not comply with town zoning rules. Instead, in an arrangement that would require the approval of two-thirds of the P&Z, Greenfield would pay a fee in lieu of parking, which would charge the commercial real estate firm $2,000 for each on-site parking spot it would have been required to provide.

P&Z Director Larry Bradley and Greenfield representatives are still calculating Greenfield's on-site parking deficit.

"Parking is the No. 1 issue because we haven't resolved it yet," Bradley told the Westport News.

The prospective redevelopment's impact on traffic at neighboring intersections also looms as a potential obstacle to P&Z approval. Rush-hour congestion already snarls nearby crossroads such as the Post Road West-Wilton Road-Riverside Avenue, although analysis by Greenfield Partners' traffic consultant, Michael Galante, indicates that National Hall, when occupied, would not overburden surrounding thoroughfares.

"I'm excited about this; I like it," said P&Z Secretary Chip Stephens. "But there are some serious things that need to be looked at in terms of traffic and parking."

Despite zoning officials' worries about parking and traffic, many town officials and downtown business owners appear to support the outline of Greenfield's plans for the National Hall complex.

"This is a very exciting plan," said Lou Gagliano, chairman of the town's Downtown 2020 Committee. "This project speaks to revitalization."

The P&Z did not vote Thursday on Greenfield's site plan for National Hall. It will continue its review of the application at a public hearing scheduled for 7 p.m. Thursday in the Town Hall auditorium.

pschott@bcnnew.com; 203-255-4561, ext. 118; twitter.com/paulschott