First of all, let’s clarify what an ADU is. An Accessory Dwelling Unit (ADU) is a second house on your property, which can be a separate suite, apartment, or even a house if you have enough space. And ADUs are becoming more and more popular as zoning and other regulations have been relaxed, as home prices continue to climb, and as the California housing shortage increases. Additionally, they can be a great investment since you can earn extra income and sell them, in certain cases, as a separate unit. Still, let’s talk first about the engineering of an ADU before you think about building one.

Like most projects, especially in the built environment, where you build something that takes up space, time, and money, you want to answer why you want to build an ADU. Is it because you want your own woman or man cave in the backyard? Or do you want to rent it out and make some cash? Or do you want to invite a relative to live in the ADU? The answer will determine the size, amenities, and layout of the ADU. Once that decision is made, you can then define the layout, including whether or not it will have its own kitchen, bathroom, or other rooms with utilities. Based on this determination, you would need to find out if the existing public utilities can accommodate the additional water and sewer demands; usually, power and telecom are not an issue. However, the “wet” utilities can be an obstacle to new development. Therefore, this part of the design must be checked early on. A qualified and licensed civil engineer can readily research and make a recommendation.

Another crucial aspect in designing and building an ADU is the soil and topography. In some cases, depending on the ground elevation, retaining walls and deeper foundations may be required. As well, the soils will determine the depth and type of footing needed. In most cases, the topography and soil type are not make-or-break factors. However, retaining walls can add cost. Therefore, it is best to address these issues early on, similarly to the utilities mentioned above.

In addition, another aspect to consider when planning for an ADU is the sustainability options, such as:

  • Solar power, which can be installed on the roof of either the main dwelling or the ADU. In any event, there are grants and payment options for installing the panels.
  • Since, in most cases, the ADU will be constructed in previously undeveloped areas, such as over a lawn or a backyard garden, the new structure will increase the amount of impervious areas. In other words, there will be more rainwater flowing to the nearest drainage system and less into the soil. Therefore, you can consider building a “green roof” over the ADU or harvesting the rainwater for irrigation and toilet flushing.
  • Sustainable materials. Now more than ever, sustainable, recycled, and or reused materials are widely available, as well as affordable. These materials include fiber concrete, ceramics, bamboo, and so on. These materials are sustainable and durable and are a great marketing feature if you decide later on to sell your property.

The Civil and Sustainable Design of ADUs