Missing middle housing—a term referring to the building type between a single-family home and a mid-rise apartment building—is increasingly viewed by policymakers, housing advocates, architects, land use planners, and developers as an incremental solution to the affordable housing crisis across the United States. However, zoning regulations, development review processes, financing structures, and anti-density sentiments, among other obstacles, have effectively made small-scale multifamily housing go “missing” from the new residential construction market.

Our new series, Unlocking the Missing Middle, will examine three aspects of middle housing. In Part 1, "Surveying Missing Middle Housing—Trends in the United States and Massachusetts," we define the missing middle building type and examine benefits and barriers to its greater production. We explore missing middle housing in Massachusetts from a historical perspective, as well as obstacles to and opportunities for creating a robust middle housing market in the state.

Figure 1: The Missing Middle Building Type Falls Between Detached Single-Family Homes and Mid-Rise Apartments

Source: Missing Middle Housing concept created by Daniel Parolek/Image ©Opticos Design, Inc./For more info visit www.missingmiddlehousing.com.

Missing middle homes are important for increasing both housing diversity and housing supply more broadly by producing gentle density in infill locations and, in the process, countering negative perceptions of multifamily housing. These types of homes encourage walkable, more sustainable living by minimizing parking requirements and vehicular dependence. They also enhance neighborhood and community livability and character by adding a diverse range of human-scaled housing types. Finally, missing middle housing offers the potential to reduce construction costs by employing simpler designs and lower-cost building materials.

Despite these potential benefits, missing middle housing faces many barriers to greater uptake and delivery, which have made this housing type more difficult to produce than it should be. Barriers include:

• Zoning requirements, such as density restrictions, large lot sizes, and outright bans of multifamily housing in many districts that have made middle housing virtually impossible to build;
• An underdeveloped economy of scale at the smaller multifamily level due to high developer fees, permitting risk and uncertainty, and lack of market comparables; and
• Community attitudes, fears, and preferences against density, perceived overcrowding, and economic diversity.

In Massachusetts, missing middle has a robust history; tens of thousands of triple-deckers and rowhouses were built in the late 19th and early 20th centuries. However, a few additional, context-specific barriers to the legalization and construction of missing middle housing today have emerged, including a complex and overlapping web of regulations, septic and energy requirements, and high construction and land costs.

While any of these barriers alone represent significant obstacles, recent regulatory changes and statewide initiatives in Massachusetts are opening opportunities for missing middle production. For example, the 2021 MBTA Communities Act introduced by-right multifamily zoning to districts within a half mile of MBTA service in the Greater Boston region; the 2024 ADU Law allowed accessory dwelling units of up to 900 square feet in single-family residential zones across the state; and the 2025 “Building for Tomorrow” Report, produced by the Governor’s Commission on Unlocking Housing Production, offered over 50 housing-positive recommendations.

This paper is the first in a series of three examining missing middle housing in the Massachusetts context. The next installment will explore case studies from other states and municipalities who have successfully implemented supportive policies, and will synthesize a small-scale housing regulatory and legislative reform framework for the Massachusetts context—so it can become a missing middle-friendly state once again.