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A rendering of the building WS Development has proposed for the Seaport District.
A rendering of the building WS Development has proposed for the Seaport District. Morris Adjmi

There’s a piece of land on Summer Street in the Seaport District that was supposed to be the site of an apartment building, bringing hundreds of housing units to a part of town that’s still finding its bearings as a residential neighborhood.

Then a fast-growing life-science company came calling. Now, the developer that owns that land wants to put an office building there instead.

WS Development is asking the city for permission to change the plan for the site — at 350 Summer St. in WS’s giant Seaport Square development — from housing to office, potentially to accommodate Foundation Medicine, which has already leased most of an office building that’s soon to break ground next door.

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While WS is also talking with other tenants as well for 350 Summer, Cambridge-based Foundation Medicine recently filed paperwork in Suffolk County reserving the right to lease space in the building.

lt’s a sign of ferocious demand for office space in the Seaport — in July, Foundation leased 580,000 square feet at 400 Summer St., and Amazon has a building underway next door — but it also shows how that demand can crowd out much-needed housing, making the goal of turning the Seaport into a bona fide neighborhood feel ever more daunting.

While WS is pledging to still build the 1,200 apartments or condos it promised when the city approved its plans for Seaport Square in 2017, they would now be smaller units and spread across two buildings in the 23-acre complex, instead of three.

That, WS wrote in a letter to the Boston Planning & Development Agency, will mean lower rents and “more smaller, lower-priced units and fewer large luxury units.”

“This shift also reflects the need for a greater density of more affordable homes city-wide, and will better serve the employment base in the Seaport District,” WS wrote.

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But critics say the Seaport needs more housing — especially at sizes and prices suited for families. The balance between housing and office space in the emerging neighborhood has been a topic of debate basically since it was first conceived a quarter-century ago.

A few large projects have been built — such as the 832-unit Benjamin and Via apartment building that opened last year, and the 717-unit Echelon Seaport complex under construction now — but larger units aimed at middle-class families are relatively scarce. Meanwhile, high-service units aimed at young workers have grown in popularity across the city. WS says its Seaport Square buildings could be designed to serve that market.

The firm’s new plans for 350 Summer St., which will need BPDA approval, would keep the building the same size — 422,000 square feet — but instead of using 350,000 square feet for housing and 72,000 square feet for retail, it calls for 384,000 square feet of office space, with retail filling the rest.

WS does not yet have a tenant signed up, but told the BPDA it “expect(s) to secure commitments from one or more major office and/or research and development tenants.” In August, Foundation Medicine and WS filed a “right of first offer to lease” with the Suffolk County Registry of Deeds, essentially guaranteeing the fast-growing genomic testing company — which is moving to the Seaport from Kendall Square — the right to lease space in the building.

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Last week, Foundation Medicine and WS filed a similar agreement for a third Seaport Square building, at the corner of Congress Avenue and Boston Wharf Road, which would only take effect after Amazon’s option to lease that same building expires next year.

There are several buildings yet to be built in the 23-acre development, two of which would now be used for housing. WS said it hopes to launch work on one of those in the next year or two.

The developer also has fleshed out a few more details of its plans, including where it will put a long-promised performing arts center.

The 600-seat SeaPAC, as WS calls it, would go in the lower floors of the building at Congress and Boston Wharf Road. If Amazon takes its option for that building, construction could start next year. In its filing this week, WS promised to fund an endowment for the performing arts center upon groundbreaking and will boost it to $8.5 million, from a previously announced $7.5 million.

WS also confirmed, in a roundabout fashion, that the $20 million in transportation improvements the state offered Amazon as part of its Seaport expansion will be passed on to the online giant, not kept by the developer. The Globe last year reported that the funds would be passed on to Amazon in the form of lower rent, though state officials have declined to answer questions on the complex deal. In its letter to the BPDA this week, WS said it “will not retain any portion” of that money.

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Tim Logan can be reached at tim.logan@globe.com and on Twitter at @bytimlogan.