Goal:

While designing the structural components of a theoretical detached accessory dwelling unit, the goal was to understand the relevant building codes and best practices, while exploring the critical design components that would be applicable to any similar construction. As advised by a wood-frame residential construction industry professional, the critical design components to consider should include lateral load calculation and resulting shear wall capability, vertical stud size and spacing, fastener type and pattern, floor assembly, and roof assembly. 

Suggestion:

To simplify the structural calculations and allow the most adaptability, a single-story, square structure, with no interior load-bearing walls formed the basis of design. The envisioned building would be 22 feet by 22 feet with 10-foot high exterior walls. A key goal maintained throughout the design process was to preserve the possibility of vertical expansion with minimal if any rework to the first floor. As advised, this approach is a common industry practice with very little additional cost for an extreme saving measure should future expansion be requested.

To once again maintain the possibility of vertical expansion, the roof support was designed identically to the first floor’s plan. In both instances, it was decided to use engineered I beams to cover the entire span. Conventional lumber would require an unrealistic size, interior load-bearing walls, or a combination.

Homeowner Consideration:

In terms of the construction process, the potential benefits and extent that prefabrication could be implemented on similar projects were analyzed.

Initially, it was hypothesized that one major hurdle for potential DADU owners would be the construction timeline in close proximity to their house. It was theorized that prefabrication could decrease that timeline and deliver a more manageable project.

However, upon talking with industry professionals, the timeline for such a small scale, the wood-framed residence would be comparable for panelized (prefabricated) methods versus stick-built (traditional). In either case, a small crew would have the framing completed in a number of days. As the much more common practice, stick-built homes require less additional planning, involve simpler transportation of materials, and are less expensive.

On a small project where the timeline is not significantly altered, the increase in price for panelized construction is simply reflected by the slightly higher quality product, assembled in a lumber yard. To the uninformed occupant though, the increase in quality would likely be unnoticeable. As more systems are incorporated into the wall panel such as electrical, or mechanical work, the price increases significantly. Again, however, on such a small scale project, each trade would require a minimal amount of time to install their respective components, not severely increasing the timeline. Additionally, when entire wall systems are installed in a prefabricated format, there is often much less flexibility for design. 

Ultimately, for a homeowner wishing to shave off a portion of the timeline by paying a higher price and limiting their design options, extensive prefabrication is an option. That option is especially attractive if a company specializing in panelized wall systems has a preset layout for the owner to select. However, for most owners that would like to have input in the design of their DADU, the traditional stick-built construction method offers a simpler, proven, cheaper means of assembly.