How Much Do ADU Plans Cost

When we talk about constructing an accessory dwelling unit, associations immediately arise about the possible generation of rental income, a comfortable reception for a guest, or the fact that projects such as ADU increase property value several times. At the same time, ADU construction is a complex process that requires responsibility already at the stage of analyzing its cost.


In this review, we will discuss the actual costs for various types of accessory dwelling units, reveal the difference between costs related to hard and soft categories, and explain why the consultation of specialists from an ADU company is necessary for successful and cost-effective construction.


Hard VS soft ADU costs

When categorizing ADU construction costs, the first thing we need to understand in detail is the difference between hard and soft costs.


Considering general ADU costs a pie, soft costs will be a tiny piece. In most cases, soft costs involve items such as average architectural and consulting costs, as well as permit costs, which we can divide into building permit fees and city fees. The amount of soft costs may vary depending on the ADU type. For example, designing a two-story garage conversion and obtaining a permit for its construction will cost more than building a small guest house.


Hard costs usually account for over 85% of the total budget, including site preparation, construction, and utilities/interior costs. From the names, it immediately becomes clear that these costs imply large-scale work, from pouring the foundation to the roof construction. They also require great physical effort and make up a much more significant proportion than soft expenses. 


Moreover, the more complex the ADU structure and the more intricate its design, the higher the hard costs.


ADU Cost and Living Structure Type Dependence

If the property owner wants to build an ADU, we recommend focusing on the range of $100-300 per square foot. However, do not take such prices as final because each ADU project is individual, especially since this residential unit belongs to the customized category.


Different ADU drafts have different price tags, so it makes sense to analyze this aspect step by step:


  • Detached ADUs cost

This is the most common ADU project, with many names, from guest houses to mother-in-law suits. Detached units are entirely autonomous, stand apart from the main house, and have different utilities and HVAC systems with them. These points affect the cost of building a detached ADU, which, on average, costs the homeowner up to $250 per square foot.

  • Attached ADUs cost

In contrast to the previous position, this project is a cost-effective ADU type. The main reason for this is that the attached ADU is adjacent to the house, which means that construction costs are automatically reduced due to the presence of a standard wall and foundation. 


If you want an attached ADU with a complete set, including a bedroom, living room, and kitchen, then the average price of such a draft will be from $125 to $225 per square foot. However, if you do not need e.g., an additional kitchen, the cost will decrease even more.

  • Garage conversions cost

The conversion of an existing structure, such as a garage, is in the middle price range. At the same time, garage conversion ADU also has a significant cost, ranging from $50 to $200 per square foot. This feature has a simple explanation; in particular, as part of the garage conversion project, you can convert the existing space into a bedroom or living room, create a home theater or gym, a hobby workshop, or a home office. The price and deadlines can vary greatly depending on the project’s complexity and construction methods.

  • Basement conversions cost

Classic or above garage ADU projects are widespread among homeowners; moreover, remember the possibility of remodeling the basement. If you own a basement, such an ADU project will cost from $50 to $120 per square foot. On par with this, you will also discover many possibilities, from creating an additional bedroom with a fireplace to a home library. If you do not have a basement, then medium costs increase due to the need for further work.

  • Prefab ADU cost

We have already mentioned that the attached unit is a cost-effective solution; however, it does not compare with a prefab ADU. Because most of the work takes place at the ADU assembly plant, the total project cost is greatly reduced, so much so that the entire draft can cost you $50,000.

ADU Costs Key Determinants

Spending on ADU development is always a very multifaceted story. Therefore, an expert can only give you an unequivocal answer to the exact budget size for constructing an accessory dwelling unit. The final cost can be influenced by various factors, from the time of year of construction to the general economic situation in the country.

We identify five key determinants of ADU costs:

  • The number of square feet. The first parameter that generates the total cost is the area of your accessory dwelling unit, or, more simply, the size of your ADU. A very simple dependency works here: the larger the area, the larger the ADU development price tag.
  • Local legislation. Each city has different rules on impact fees, property taxes, or zoning laws. All these positions have a direct impact on the overall cost.
  • Geographical disposition. The construction costs of the ADU vary depending on its location in the city. For example, an ADU near urban infrastructure is cheaper due to communal nodes, unlike an ADU in a vacant lot.
  • The type of assembly. The biggest difference in ADU expenses is between custom and prefabricated ADUs. The latter will cost the property owner many times cheaper than designer construction.
  • Additional chips. Every homeowner has their idea of a dream ADU. For some, this is a living area with a fireplace and a library; for others, it is the presence of a home theater; and for others, world-class plumbing. In any case, the more amenities you finish, the higher the total cost will be.

Overall, all these factors are quite difficult to manage in your head, so we recommend consulting with specialists to manage your construction budget more effectively or even save money.

ADU Cost & Professional Assistance

At first glance, analyzing ADU building costs seems like a simple task. However, in this case, you can not do it without the help of professionals, and there are four strong arguments for this. Firstly, specialists can decompose the project’s total cost into the smallest details, for example, telling you what proportion of site preparation costs will be and what percentage will be for costs excluding labor.

Secondly, professionals are aware of all the rules of the local planning department, which means they can only tell you about the city fees and permit costs that you should follow for the legality of construction. Thirdly, suppose you are experiencing a problem with financing. Analysts from the general contractor’s teams will help you pick the best option and tell you the difference between a home equity line and a construction loan.

Finally, when we talk about ADU building, we should mention “hidden costs.” These costs can stretch the budget very much in some cases, so if you want to avoid unexpected expenses, then the help of professionals in financial planning will definitely not be superfluous.

Frequently Asked Questions — FAQ

What is the difference between soft and hard costs?

Hard costs include expenses for physical labor, particularly construction, site prep, and interior costs (they account for about 90% of expenses). Soft costs consist of three parts: consulting, permit, and architectural design costs, which usually account for 10% of all expenses.

Why contact specialists if I can calculate actual expenses for construction myself?

Construction includes many actual, hidden, and potential costs that only a pro can determine. Experts will provide detailed cost estimates and information about the actual cost per square foot for each ADU type, help you get permission, and save money where you do not suspect it.

What are the associated costs of construction?

The associated costs are expenses that do not directly relate to the construction process; however, it is only possible to complete the project with them. For example, permit costs are those without which construction will become illegal, and consulting costs are those without which construction will be ineffective.

Why is the appearance of ADU on my site a good investment?

If you want to build an ADU, then be prepared that this solution will bring you only advantages. In particular, it provides the opportunity to receive passive income, equip a home office, comfortably accommodate elderly parents, or receive guests for a long time. The most crucial thing is to approach construction wisely, already at the cost planning stage.