Beginning with the definition of such:

"Missing Middle” is a term that refers to the range of housing types that fit between single-family detached homes and mid-to-high-rise apartment buildings. Used in this context, “middle” references the size and type of a home, and its relative location – in the middle – on a spectrum of housing types. These housing types are commonly house-scaled buildings, yet with more than one unit. Examples include duplexes, triplexes, townhomes, and others that will be discussed in this report. The cost of these housing types varies based on style, size, location, and market forces. Missing Middle (MM) housing types do not always correlate with a specific income bracket but can be less expensive than other housing options that are larger and take up more land.

It is somewhat inaccurate to equate "missing middle" with middle-class housing (as I have done a few times), but the correlation with median income is not too far off the mark. Understand, much of this study involves "taking the pulse" of existing community members, and you will see a lot of common complaints (traffic, overcrowded schools, loss of green space and canopy). I won't say these are not legitimate complaints, but I will say that many citizens use them as a crutch when they are really concerned about "those people" moving in. Here are the main choices Arlington found to increase the missing middle:

Small lot single detached – a small single detached home, possibly more energy efficient and accessible, easier to maintain, and less expensive than larger single detached homes; compatible in height and scale with other smaller single detached homes; found interspersed in Arlington’s low-density residential areas.

Duplexes (stacked) – looks like a single detached home; compatible in height and scale with single family homes; commonly found interspersed in Arlington’s low-density residential areas; ground floor units can be accessible. Duplexes (side-by-side) - compatible in height and scale with single detached homes; commonly found interspersed in Arlington’s low-density residential areas; provides for living style and design comparable with single detached home.

Triplexes and Fourplexes – can be designed to look like a single detached home; compatible in height and scale with single detached homes; found interspersed in Arlington’s low-density residential areas; may be less expensive per unit than other options due to smaller land requirement and smaller unit sizes; effective transition from mid-rise development to lower density residential areas.

5-8 Unit Buildings – commonly found in Arlington’s garden-style apartment communities and in some low-density residential areas; compatible in height and scale with large single detached homes; may be less expensive than other options due to smaller land requirement and smaller unit sizes; effective transition from mid-rise development to lower density residential areas.

Townhouses (side-by-side) – provides for living style and design comparable with single detached home; found interspersed in Arlington’s low-density residential areas; if designed efficiently, may be less expensive
than other options due to smaller land requirement; if designed without front-facing garages, interesting, lively, pedestrian friendly street front; effective transition from mid-rise development to lower density residential areas; opportunities for rental units (basement/ground level) to offset housing costs.

Townhouses (stacked) – provides opportunity for larger units than a small multiplex yet may be less expensive than other options (i.e., duplex) given smaller land requirement per unit; option to create interesting, lively,
pedestrian-friendly street front; effective transition from mid-rise development to lower density residential areas.

We've had some successful infill projects on the "small lot single detached" front, often involving subdividing large older lots. One in particular featured 5 new single family homes that sold in the $150,000 range. We've also had a lot of growth in side-by-side townhouses, but the average price has creeped up from $250,000 to $350,000, and rising. I would love to see more 2-3-4 plexes, but landowners haven't expressed much interest. And when they do, the torches and pitchforks come out.

What they elected to not include in this study:

Accessory Dwellings are not recommended to be studied in Phase 2. Accessory dwellings are already permitted through recent changes to the Zoning Ordinance and an extensive community process.
While not enabled on all lot types and with the design flexibility that may be most desirable, recent changes to accessory dwelling regulations are allowing the creation of a Missing Middle housing type that meets several key community priorities and addresses community concerns, including but not limited to achieving greater housing choice, reducing housing costs, and compatible integration into existing communities. Permitting accessory dwellings with other Missing Middle housing types may be explored.

Cottage Clusters are also not recommended for study in Phase 2. Due to lot size requirements, integrating this housing type into the existing Arlington context may be more difficult than the other housing types
under consideration. However, exploration of small lot single detached homes could enable development of recommendations for how existing regulations for residential clusters could be modified to support cottage
clusters with smaller lot and dwelling sizes.

Just a little personal preference: I can see myself living in an accessory dwelling unit, if my privacy is respected. But the idea of living in a cottage cluster, with everybody being up in everybody's bidness, kinda gives me the crawlies. Your mileage may vary.

I will leave you with some advice I gave my fellow Planning Board members on more than one occasion: Just because a supermajority of people who actually show up at a zoning hearing feel a particular way, the decisions should not hinge upon that. Give those opinions respect, but not undue influence. Having a diverse community reaps benefits that many people cannot (or will not) acknowledge, and all growth generates some discomfort.