Cost breakdowns, the 2025 and 2026 rule changes, permit timelines, and the rental math, from a crew that designs, permits, and builds ADUs across Whatcom County.
Real 2026 ranges by type, from $80,000 for a garage conversion to $400,000+ for a detached cottage, plus the line items that move the number.
$80,000 to $150,000 in 2026. Why a detached garage runs 20% to 30% more, what is included, and how to read a quote.
$250,000 to $400,000+ at roughly $400 to $550 per square foot. The line items behind a ground-up cottage and what drives the range.
Two ADUs per lot, a 1,000 square foot cap, 24 feet of height, and no parking minimum. BMC 20.10.036 in plain English.
The owner-occupancy requirement sunsets July 1, 2026, so most city lots can rent both the house and the ADU. What that changes.
The 6 to 12 month timeline, the City versus County path, plan review, inspections, and the things that quietly add weeks.
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