Tandem House vs. Accessory Dwelling Unit

Aiding in our design process from concept to final design, Scott and I often look for precedents as an inspiration to help guide the decision process and methods to employ in our house design.  These precedent studies can help refine the character of the house and also determine certain aesthetic elements.  Sites like Houzz, Architizer, and Dwell Magazine are great resources.  Never mind the inspiration we draw from professional practice and observation of new design and construction in Denver.

While design inspiration may be abundant, there are very few instances of examples of the tandem house form in Denver.  We have scoured neighborhood alleyways, internet sites like Zillow, and have had an open dialog with allied professionals and the City; and yet through all this research, thus far, have found only a handful of tandem house precedents in Denver.  Even when we went into the City development office to review preliminary drawings, it was apparent that they had not seen many tandem house permit applications, as few employees were fluent in this building form.

Further, many times when we discuss the building of a second home off the alleyway of our lot, the common response is “Oh! An ADU!” or “Oh! A Carriage House!”  And even though people are aware of Accessory Dwelling Units (ADU’s) and Carriage Houses, it has taken the ever increasingly competitive housing market and demand for affordable child and senior care options to create a pent-up an interest.  The movement still has yet to take off, despite it being an allowed building form for nearly 7 years now. And, the short-term rental movement (made possible through sites like Airbnb) has only begun to benefit from the ADU market in Denver.

ADU’s and Carriage Houses definitely have their fair share of benefits but they can also, in many ways, be restricting and not satisfy every homeowner’s needs.  In our case, the Tandem House form is more appealing alternative. Below are key comparisons:

ACCESSORY DWELLING UNIT (ADU):

Building Form Intent: Establish standards for one primary dwelling structure and one accessory dwelling structure on a single zone lot; where the zone lot owner is required to occupy one of the units and must own both structures.

Maximum Building Footprint: Zoning requires that the maximum footprint is limited to 1,000SF (as opposed to the max building lot coverage that is more flexible with the tandem house building intent).

Maximum Building Height: Limited to 24’ defined by 1.5 stories high.

Payback Potential: The Accessory Dwelling Unit cannot be sold; only rented. Return on investment can be translated into an overall increased home value and generated monthly income from renting it.

TANDEM HOUSE:

Building Form Intent: Establish standards for two primary dwelling structures on a single zone lot; intending that the two primary structures can be legally sold separately and owned by two separate entities.

Maximum Building Lot Coverage: Zoning limits the total building coverage on a single zone lot to 37.5%. This gives flexibility to the building footprint of a second primary structure when a primary structure already exists. Say, for example, the existing primary structure at the front of the lot covers 15% of the zone lot, the second proposed  primary structure in the rear can cover the remaining 22.5% of the zone lot; potentially allowing for a larger footprint if setbacks allow.

Maximum Building Height: Limited to 24’ and 30’ (depending on where the structure sits on the lot) defined by 2.5 stories high.

Payback Potential: The tandem house could be sold or rented. If sold, the return on investment can be gained back immediately.  If rented, monthly generated income could be higher compared to an accessory dwelling structure with fewer amenities (fewer bedrooms and bathrooms and less living space).  You may also decide to rent initially and later sell if circumstances change.

Both building forms create more options for homeowners and it is important to compare the two if your lot is zoned for it.   Whether it is more space for a growing family, to cover monthly mortgage payments, increase home value, or to immediately gain a return on an investment. No matter the attractiveness, Tandem Houses and Accessory Dwelling Units create more housing options in a city with serious affordable and market rate housing deficits.

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Soil Test and Geotechnical Investigation

Although we are still waiting for the results of a geotechnical report, we were able to check Soil Testing off the list. The report that comes back must accompany the construction documents that we submit to the City for permitting. Two boring locations were drilled with a 4-inch diameter rig, powered by a 140-pound hammer that goes down to 25′ below the surface!

 

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Samples were taken at select intervals and material will be logged to evaluate the engineering properties of the soil layers that include swell consolidation testing and moisture-density determinations. When we get the geotechnical report back Scott will use this to determine the design of the foundation and appropriate structural load reductions.

Taking this step made the process feel very real. Like this is really happening! The neighbors must be a little curious since we had to take the fence down in the back to get the truck in. And, it’s difficult to ignore the shaking as the rig pounds into the ground!