Construction costs for your ADU will vary significantly depending on personal preferences, site conditions, location, and many other factors.

Size: Despite what many think, smaller ADUs may cost almost the same as larger ones. Many costs like foundation, kitchen and bathroom work only increase slightly for larger ADUs. Kitchen costs will range from $25,000–$50,000 with each bathroom ranging from $15,000–$25,000.

Type: New construction, both detached and attached, tend to be the most expensive. Garage conversions are not much cheaper than new construction if at all. Conversions of interior space (basement or otherwise) are often the cheapest.

Other factors:

• Quality of interior finish work and amenities
• Architectural form and details
• Extent of utility, structural, mechanical, electrical, and plumbing upgrades required
• Required site upgrades (sidewalks, sewer and water)
• Whether sprinklers are required
• Whether doors and windows meet emergency exit standards
• Lot complexity (slope, trees, fault lines, etc.)