The Thunderbird Motel was a longtime eyesore, even among the notorious stretch of seedy establishments along Aurora Avenue. The gun violence, drug dealing and prostitution were so thick, the building was declared a public nuisance, a title shared by few and coveted by none.

But earlier this summer, on the spot where the Thunderbird once stood, an affordable housing project called Patrick Place opened -- a collection of small studio apartments with a garden, a common room, laundry and even a roof planted with grass. Forty of its units will be for people earning less than $750 a month; the other 30 will be for the formerly homeless or shelter-bound.

At a time when Seattle is seeing more and more of its low-income residents driven out, the Thunderbird's transformation offers a look at how housing can be built for those most in need. But the fact that it took four years to get there hints at a hard truth: Building truly affordable housing in this city is incredibly difficult.

Mayor Ed Murray’s Housing Affordability and Livability Agenda (HALA) taskforce has set out to add 20,000 affordable units over the next ten years, a two-thirds increase from Seattle’s current stock. Just about everyone agrees that even 20,000 units falls short of the city’s true need, particularly for those in the lowest income brackets. Nonetheless, Murray says the goal is a “stretch.”

Why is it so hard to build low-income housing? The easy answer is because of money, or lack thereof. But time, space and bureaucracy loom large as well.

The first challenge to building affordable housing is fundamental to any real estate deal: location. According to Scott Starr, an architect with SMR, a firm that designs low-income apartment complexes, the ideal size for a low-income housing project, from an efficiency standpoint, is between 70 and 80 units (although members of the Low Income Housing Institute pointed out that they build projects with 50 units). The only places you’re allowed to build structures that large are downtown, in urban villages and along transit corridors -- at least two of which happen to be where for-profit real estate developers want to put their buildings as well. In other words, during a housing boom, when low-income housing is needed most, it gets even harder to make it happen.

Patrick Place, a project of Catholic Housing Services, benefited from starting during a development bust, when competition for both space and construction firms was mild, Starr says. Had the project begun now, he says, "construction cost would definitely be higher. We’re competing with the market rate, multi-family developers. We have to compete with those guys for everything."

There’s also the matter of getting approval for these projects, which often prompts resistance from neighbors concerned about low-income tenants. Patrick Place got some pushback because the building was two stories higher than the Thunderbird, but because of the motel's ugly reputation, neighbors largely cheered the transformation. (One man, though, bought the Thunderbird sign for nostalgia's sake and is restoring it in his garage.)