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There are six (6) major variables that affect an accessory Dwelling unit’s construction cost:

-Home Size

-Location

-Design

-Lot cost

-Utilities and fees

-Production & Repetition

Home size:

A median sized 2400 ft2 wood framed one-story home costs an average of $83.15 per square foot to build. However a small 600 square foot one story home costs $133.40, a difference of $42.15 per square foot or 50% more for the smaller house.

Location:

Home construction cost varies across the country with lower prices in Texas, Florida and the Deep South. Homes on both coasts will average higher. Variations between locations, for the same design, can be as much as $90.00 per ft2.

Lot Cost:

While lot or property price is not included in any of these numbers, the construction cost is affected. It's subjective, but homes on expensive property usually employ higher quality materials and better craftsmanship in order to justify the total cost. The converse is also true, a home on less expensive property is usually built with less expensive materials and methods. The test for this decision is at the time of resale.

Design:

A 1,000 square foot two story home costs $111.55 per square foot while its 1,000 square foot one story home counterpart costs $118.25, a difference of $7.70 per square foot. I've seen differences of up to $20.00 per ft2 and a $10.00 difference is common.

Utilities and Fees:

Water service in some areas may cost as much as $5,000 or $10,000 and even more just for the right to have water service. Fees and regulations can total more than $50,000 for each new home. $50,000 in fees for a 3,000-ft2 home adds $16.67 per square foot, but in the case of a 600 ft2 home $50,000 represents $83.00 per square foot. To help nulify this inequity some cities reduce rates for ADUs or allow service connections to the primary house which may cost nothing in fees.

Production & Repetition:

Building one house at a time is more costly than building a large quantity of the same plan over and over.

The typical one of a kind home in my city, Denver, costs approx. $200.00 per square foot. Part of that is in upgraded materials but much of that is offset by the less costly large size of most custom homes. A production home including the same items may cost as little as $90.00 per square foot or less than 50% of the custom home cost. Every carriage home or Granny Flat in the backyard is essentially built as a one-of-a-kind home.

Summary:

Many of the above items can be added together to get the total price for your area. I got all of the cost data from R. S.Means 2009 edition of their residential cost estimates. I hope this exercise gives you a better feeling concerning the things that impact the cost of a new ADU or Accessory Dwelling Unit.

Mike Kephart

Sidekick Homes

www.sidekickhomes.com