In order to evaluate the construction and material costs of DADU’s, we used the extensive research of Dr. Kol Peterson, the organizer of the largest additional dwelling unit (ADU) tour in the United States and the author Backdoor Revolution: The Definitive Guide to ADU Development.  Dr. Peterson collected self-reported cost data from multiple ADU owners for manufacturing DADUs, including design costs, permit costs, utility connection costs, and all construction costs.

Table 1. Average Material and Construction Costs for Three Distinct Attached Dwelling Units in Portland, Oregon. n = 30 for (1), n = (2),  n = 6 (3). 

After analyzing the data, it is discernible to see that, despite the size of the units, there is a wide range of development costs. Dr. Peterson suggests that “side of conditions, level of finish, and overhead costs of the designers and general contractors” are factors that affect the development costs. Furthermore, it makes common sense that the size of the DADU matters but not to the extent that one may initially surmise- this stems from the fact that the fixed costs of the DADU’s are expensive (i.e. excavation costs, concrete framework, foundation pouring, etc.) but the marginal cost of additional square footage is not.