A developer wants to build more than 300 housing units off Spring Street in Westbrook – right next to where it is already building nearly 200 single-family homes and apartments.

Blue Spruce Farm’s 500-some units would put it among the largest residential developments in the region. It would dwarf Dunstan Crossing, the planned neighborhood in Scarborough with more than 250 homes, and even be larger than the mixed-use “midtown” development, with 440 market-rate apartments proposed for Bayside in Portland.

With housing in high demand and few neighbors to appease, the massive project could help to quickly fill the region’s residential shortage.

“I believe an increase in supply is the most rational way to stall the dramatic increase in rents,” said Caroline Paras, economic and community planner for the Greater Portland Council of Governments.

Developer Risbara Bros. has all the proof it needs that the demand is there.

Just a year after the Westbrook Planning Board approved the project’s first phase, for which Bill Risbara predicted a three- to five-year buildout, all 53 of the single-family home lots have sold, 50 apartments are occupied and tenants are waiting to move into others that have yet to be built.

Risbara said Monday he’s confident that all 189 units will be filled by Christmas.

The success of the first phase prompted Scarborough-based Risbara Bros. to purchase 50 adjacent acres, which it has under contract.

On Tuesday, the developer will present a site plan to the Westbrook Planning Board for a second phase that includes 16 apartment buildings with 12 units each, as well as mixed-use buildings with first-floor commercial space, condominiums, single-family homes or duplexes, and a dog park.

Risbara said the project is “full steam ahead,” although it will probably take six to eight months to receive local and state approvals. He said construction could start next spring.

The project is the biggest Risbara has done and would be the biggest the city has seen.

The first phase alone was the second-largest development in Westbrook history, smaller only than a subdivision of more than 200 single-family homes built off East Bridge Street in the 1960s and 1970s.

Risbara has no concerns that it’s overly ambitious.

“We feel very comfortable with the market,” he said.

He said the project hasn’t had a major impact on Westbrook schools, where enrollment has been rising recently following years of decline. After closing one of four elementary schools in 2012, the district, which has more than 2,500 students, is now considering expanding another elementary school and its middle school.

Risbara said he knows of 10 school-aged children and 12 toddlers or babies in the single-family homes, and one infant in the apartments.

The project is capitalizing on two trends – a boom in home building this year to pre-recession levels, and a shortage of rental housing in Portland that’s forcing tenants to look outside the city.

In the first quarter of this year, the number of new housing starts in Greater Portland was nearly equal to those in the first quarter of 2006, before the recession halted construction.

Westbrook led the way with 29 new homes, up from eight the year before and surpassing any year in at least the past decade, probably because of the Blue Spruce Farm project.

Portland’s rise in popularity has resulted in a vacancy rate near zero for apartments, and rents that have shot up about 40 percent in the past five years.

“There’s such a need and such a demand that they could build out all of their land and we could build out all of the golf course and there would still be a demand,” said Jim Howard, whose Priority Real Estate Group plans to close Wednesday on the 52-acre Twin Falls Golf Club across the street from Blue Spruce Farm.

While Portland feels the brunt of the housing crunch, it’s creating opportunities for other communities.

“As everybody knows, the peninsula is full,” Howard said.

Although he doesn’t have specific plans, he believes the golf course’s mature trees and nearby Stroudwater River create an attractive landscape for upscale apartments and single-family homes. It helps that major employers, namely Unum and Idexx Laboratories, are nearby.

New construction is also always a selling point, said Brit Vitalius, president of the Southern Maine Landlord Association.

The single-family homes in the first phase started at $225,000, and rents start at $1,100 a month.

Vitalius said he’s not surprised that Blue Spruce Farm has done well, and he hopes its success continues.

But he also hopes it doesn’t oversaturate the market and discourage residential investment in Westbrook’s downtown.

“From my conservative Mainer perspective, that’s a lot of units,” he said.