Prefab vs. Stick Built ADUs: Which is Right for Your Property?

Accessory Dwelling Units (ADUs) are quickly becoming one of the most practical ways to create additional housing in Greater Boston, whether you’re planning space for family, rental income, or future flexibility. But one of the first questions that comes up is how to build it. Homeowners have two primary options on how to build their ADU, prefabricated or stick built.

Prefab ADUs fall into two categories, manufactured and modular. Both are constructed off-site before being transported and installed on your property. Stick-built ADUs, by contrast, are built entirely on-site using traditional framing and finishing methods. Each approach carries its own advantages and tradeoffs in terms of cost, schedule, customization, and permitting.

This guide outlines how both types of ADUs are built, what to expect from the process, and which factors matter most when deciding which method fits your property and goals.

ADU Build Types: Prefab ADU vs Stick Built ADU in Boston

There are several ways to build an accessory dwelling unit (ADU), and understanding the construction methods is the first step in choosing what’s right for your property. Broadly, ADUs fall into two categories: prefabricated (“prefab”) and stick-built. These terms describe how and where the unit is constructed.

Prefab ADUs are built at least partially in a factory before being transported to your property for installation. This category includes modular and manufactured ADUs. Modular units are built to the Massachusetts State Building Code, while manufactured homes follow federal HUD standards. Both aim to reduce waste and limit weather delays.

Stick-built ADUs, on the other hand, are constructed entirely on-site using traditional framing methods, usually wood, or “sticks.” Each wall and roof section is measured, cut, and assembled directly on the foundation. This approach offers the most flexibility in design which makes it a popular choice for projects that need to match the style of an existing home or meet specific design requirements.

In short, prefab construction prioritizes speed and cost, while stick-built construction emphasizes flexibility and long-term value. Each approach has clear advantages depending on your goals, property layout, and timeline.

Here’s how these build types compare in more detail:

Manufactured ADUs in Boston

Manufactured ADUs

Manufactured homes were originally known as “mobile homes,” a term used for RV-style housing designed to be moveable. Today’s manufactured homes have grown in complexity but are still built fully off site in a warehouse/factory and are delivered as one completed structure ready for use. Most manufactured homes today are placed on a permanent foundation instead of being mobile.

Unlike modular or stick-built ADUs, manufactured homes are built to federal HUD standards rather than the Massachusetts State Building Code. This difference disqualifies them from being considered legal accessory dwelling units in Massachusetts. In order to receive a building permit and certificate of occupancy in Massachusetts the structure must be built to state building codes.

Below we outline the pros and cons of manufactured homes but since they do not qualify as legal ADUs in Massachusetts, the remainder of the article will focus on modular ADUs.

Pros of Manufactured homes

• Quick installation
• Minimal site disruption
• Economical option for temporary housing or guest accommodations

Cons of Manufactured homes

• Built to HUD code, not Massachusetts residential code, which limits where they can be permitted
• Do not qualify as a legal ADU in Massachusetts
• Often ineligible for conventional mortgages or home equity financing
• Limited customization options for layout or finishes

Modular ADUs

Modular ADUs are built in a factory setting in boxed sections or “modules” that are then delivered to the property and maneuvered onto a permanent foundation with a crane. Typically a modular ADU will come in 2 or 3 boxes that are set together to form the structure.

The modules are framed in the factory and come with windows, exterior doors and exterior sheathing already installed. The interior utilities (HVAC, plumbing, electrical) are also installed in the factory beforehand. Then once the home is set the roofing shingles, exterior siding and interior finishes must then be completed on site by local laborers.

Unlike manufactured units, modular ADUs must meet Massachusetts residential building codes and local energy efficiency standards. This type of construction makes modular ADUs eligible for legal ADUs in Massachusetts. These construction standards also offer increased long term value and use for the homeowner.

Cost of modular ADUs

Many homeowners assume that choosing a prefab modular ADU will lead to significant cost savings. However, after comparing actual costs, we’ve found that the final prices often end up surprisingly similar. Much of the same work still needs to be done on site: permitting, excavation/foundation, utilities and finish work. While the remaining work is done in a factory setting, the cost of materials does not change significantly and employees are still needed to perform the work. Delivery and crane operation also need to be conducted in order to set the modular ADU.

We found more significant savings in smaller modular ADUs vs traditional stick-built than with larger units. It is important to ensure all aspects of construction, namely plans, permits, sitework, utility connections, delivery, finish work & finish materials are included in the price quote. See below for a breakout of modular vs stick-built ADU costs.

Speed of construction

We also reached out to a few modular homebuilders who offer an ADU product and were given an 8-12 month timeline from signed contract to project completion. This is in line with what stick-built ADU builders will deliver. Although modular homes are built in a controlled factory setting which lessens the actual build time, the factories are often backlogged and cannot begin work as soon as an order is placed. This results in a similar total project time frame to the ADU being built on site using traditional methods.

Customization

Many modular ADU manufacturers offer a range of customization flexibility. Floor plans, roof lines and window/door placements can be adjusted to meet specific design preferences. There are some limitations due how each module must fit on the delivery truck but these are normally not overly restrictive.

However, the placement of the modular ADU is where issues can arise. There needs to be at least a 30x30 foot open space to park the crane in order to lift the modules to the rear of the property. Other factors like power lines, trees and adjacent buildings can also restrict installation. These logics can be tough to navigate especially on tighter lots like those found in Greater Boston. It is not uncommon for a modular home manufacturer to turn down work due to difficult site logistics.

Financial and Long term value

The best way to maximize value when building an ADU is to have it match the existing home and to optimize its location on the lot. While modular ADUs are quite versatile, there are instances where there can be limitations with customization and lot placement. This could affect the long term value it adds to the property. It could also hinder a homeowner’s ability to get a loan for the project as banks often factor in the value of the property with the ADU when calculating a loan.

As modular homes and ADUs have become more common, most banks are familiar with this construction type and . However, depending on the bank you are using and their sophistication, there may be additional hurdles when financing an modular ADU. See our full guide on how to finance an ADU in Massachusetts here.

Quality Control

The opinion on the quality of modular ADUs can differ depending on who you speak to. Some argue that because they are built in a factory setting, protected from weather conditions and operated like a production line that the quality is superior than if built on site at the property. Others argue that many of the factories are overbooked, poorly run and you don’t know what you are getting until it is delivered.

Our advice: if you choose to build a modular ADU, do your due diligence. Visit the factory if possible, meet with the production team and understand what you are paying for. Also, reading reviews and getting references from other buyers is always advisable with high priced purchases like these.

Stick-built ADUs

At Horizon ADU we choose to build our ADUs using traditional stick-built methods. Our experience and local network allow us to build an outstanding product that is fully customized to your needs. We prefer the ability to perform quality control on a day to day basis instead of trusting things are being done correctly in a factory. This approach also allows us to build ADUs on tighter urban lots that may not be accessible for crane delivery of a modular unit.

Pros of Stick-Built ADUs

• Fully customizable to match your existing home.
• Built to Massachusetts residential code and easily permitted in Boston.
• Long lifespan and higher resale value.
• Easier to modify during construction.

Cons of Stick-Built ADUs

• Potentially longer construction timeline and cost
• Susceptible to weather delays
• Requires local expertise

Choosing the Right ADU for Your Boston Property

The best ADU construction method depends on your property type, budget, and long-term goals. Modular ADUs are fully code-compliant and can potentially be faster and less expensive depending on your lot. Stick-built ADUs provide full creative control and long-term property value.

Key Takeaways

• Manufactured ADUs are not considered legal ADUs in Massachusetts because they are not built to state building code
• Modular ADUs can be slightly more affordable and faster than a stick-built ADU but can pose challenges with customization and property logics especially on tiger lots
• Traditional stick-built ADUs