During the last month, more than a dozen people have written me expressing concerns about nearby developments with high-density housing. They felt it might contribute to flooding their properties. They may be right. But the story is not simple. Many people see benefits, too. Whether you are for or against such developments will depend on circumstances and your point of view.

During the last three decades, the homebuilding industry has seen a trend toward dwindling lot sizes. As lots have shrunk, the percentage of lots occupied by homes has grown. We are now at the point where developers will need a shoehorn to squeeze homes onto lots. Nationally, Texas has the smallest lots with the exception of the Pacific Coast. As one looks at these new smaller lots from the air, it’s hard to see where one could squeeze in a tree. Growth of impervious cover, one factor that contributes to flooding, staggers the imagination. What’s driving this trend? And is flooding an inevitable consequence

Driving the Trend: Affordability

According to the Department of Housing and Urban Development, a combination of housing underproduction and higher consumer demand, particularly among millennial first-time homebuyers who delayed household formation as a result of the 2008 recession, is contributing to rising housing costs. 

Significantly, the cost of entry-level homes has risen much more sharply than overall home prices or the prices of luxury homes. Even when first-time buyers can purchase a new home, they increasingly buy farther from city centers. This trend can impact the amount of time people spend commuting and influence regional infrastructure needs.

Further, the number of cost-burdened owners (those paying more than 30% of their income on housing) has receded to pre-2008 levels, whereas the number of cost-burdened renters remains close to peak levels.

Buyers More Willing to Sacrifice Lot Size than Home Size

Builder Magazine cited a study by Freddie Mac and the National Association of Homebuilders (NAHB). It found that reducing home buyers’ spending on land, rather than housing, is one method to improve housing affordability.

Research published in 2017 by the Federal Reserve shows the median size of a single-family home built from 1980 to 2014 grew by 50 percent, but the median lot size decreased by more than 20 percent during the same period.

In other words, builders are squeezing bigger homes onto smaller lots.

Builder Magazine

A graph from a Federal Reserve Board study dramatically illustrates these trends.

Lot Sizes Hit Record Low in 2019

According to the NAHB and US Census Bureau, median single-family lot sizes have hit a record low.

Regional Variation in Lot Sizes

The same article points out that some of the smallest lots can be found in Texas, a state with almost unlimited amounts of land.

Land Costs now 39% of Building Costs

Land costs largely drive these trends. NAHB says that, on average, 55.6 percent of the final sales price of a new home goes to construction costs and 21.5 percent to finished lot costs. While that’s less than a quarter of the total home cost, it’s 39% of construction costs. The NAHB shows that land costs are the single largest cost component of a new home (largely because construction costs are broken down into smaller categories, such as contractors, materials, etc.).

As a consequence, developers are packing homes into lots tighter than sardines. See the photos below.

Two Porter Examples

The average lot size in Porter’s Brooklyn Trails Development is .12 acres (about an eighth of an acre). The homes range from 1,307 to 2,628 SF. The builder aggressively markets them to first-time buyers stepping up from apartments by promoting “closing cost assistance,” “free washer, dryer, fridge,” and prices starting from $170,000.

Compensating for a Higher Percentage of Impervious Cover

According to Matt Zeve, deputy executive director of the Harris County Flood Control District, hydraulic models used to calculate detention pond requirements in such developments typically factor in the percentage of impervious cover. So do most flood plain regulations.

However, in the case of Brooklyn Trails, I discovered via a Freedom of Information Act Request to Montgomery County that the developer filed its application for a building permit two weeks before new Atlas-14 rainfall frequency estimates went into effect. This was another case of “beat the clock.”

As a consequence, Brooklyn Trails will only have 60% of the detention pond capacity needed for this area. They got to define the 100 year/24 hour rainfall as 10″ instead of 17.3″. A smaller detention pond means more buildable lots.

Buyers will only pay the upfront costs. Neighbors and downstream residents will pay the backend costs – in flooding. This is bad. But the badness stems more from inadequate detention than lot size.

Three Recent Developments in Spring, TX

Many Governments Use Regulation to Reduce Impervious Cover

Google “flooding” and “lot size.” You will find thousands of articles and regulations from across the US. Most see regulation of minimum lot size as a tool to reduce impervious cover and therefore flooding. Rhode Island, for instance, says “Under natural forested conditions, only about 10% of precipitation runs off the surface of the site, 50% soaks into the ground, and a surprising 40% is taken up by trees and other vegetation and sent back into the atmosphere through the process of evapotranspiration.” Total runoff volume for a one-acre parking lot, they say, is about 16 times that produced by an undeveloped one-acre meadow.

New York also recommends larger minimum lot sizes to reduce the number of building lots that may be created, providing greater area for natural systems to process stormwater and reduce flood risk. They also advocate “maximum lot coverage standards.” That helps explain why land-starved New England has the largest minimum lot sizes in the country – .6 acres (see US map above).

But the story is a little more complicated than just reducing the amount of impervious cover. With sufficient, mandatory detention and enforcement of regulations, theoretically, developers could offset the volume of water soaked up by all those trees and grasses.

High-Density Developments Have Benefits, Too

In addition to lower home costs, high-density developments offer several other benefits. You may or may not value or agree with.

Higher density uses infrastructure more efficiently. For instance:

• One fire station could cover two or three times as many homes without compromising response time.
• You can also pack more homes on a street; that uses less concrete for streets.
• Smaller lots mean more homes on available land, which generally increases tax revenues for cities and counties.
• They also limit urban sprawl, which can preserve floodplains beyond the reach of the City.
• Less sprawl also means less commuting, which reduces energy consumption and gives people more time to spend with families.
• Higher density creates tighter neighborhoods, where people interact more with each other.
• And finally, higher density encourages more walking, which leads to healthier lifestyles.

We need more research to quantify these tradeoffs. In the meantime, “dwindling lot size” doesn’t automatically go into the win or loss column. Smaller lots have value, just as they have drawbacks. The real issue has to do with building enough detention to offset the high rates of runoff. And whether you still have a Millennial living in your spare bedroom.

Posted by Bob Rehak on October 11, 2020

1139 Days since Hurricane Harvey

The thoughts expressed in this post represent opinions on matters of public concern and safety. They are protected by the First Amendment of the US Constitution and the Anti-SLAPP Statute of the Great State of Texas.