Most homeowners know they can build one ADU on their property. Fewer realize that Bellingham allows two. Under Washington's HB 1337 and Bellingham's municipal code, residential lots can support up to two accessory dwelling units in any combination of types — detached, attached, basement, or garage conversion.
Two ADUs on one lot can generate $33,000 to $53,000 per year in rental income. That is a genuine second income stream from property you already own. Combined with property value increases, tax benefits, and multigenerational housing flexibility, a two-ADU strategy transforms a single-family lot into a powerful financial asset.
This guide covers the legal framework, the best combinations, income projections, phasing strategies, and which lots are best suited for multi-ADU development. If you want to know whether your specific lot can support two ADUs, our free feasibility study will calculate exactly what is possible.
The Legal Framework for Two ADUs
The right to build two ADUs is established at both the state and city level:
- Washington HB 1337: Signed into law in 2023, this requires all cities with populations over 25,000 to allow at least two ADUs per residential lot. Cities cannot impose owner-occupancy requirements, cannot require additional parking, and must process ADU permits administratively. Full HB 1337 guide.
- Bellingham Municipal Code 20.10.036: Bellingham's ADU ordinance implements HB 1337 and permits any combination of ADU types: two detached, one detached plus one attached, detached plus conversion, or any other pairing. Bellingham ADU laws.
- Any combination of types: There is no restriction on which types you combine. You can build two DADUs if your lot supports it, or one DADU and one basement conversion, or one attached and one garage conversion. The flexibility is designed to let homeowners choose the combination that works best for their property.
The practical constraints are not legal — they are physical: lot coverage, setbacks, and utility capacity. These vary by zone and lot. Our feasibility study calculates exactly what your specific lot can accommodate. For a deep dive on the two-ADU rule, see our dedicated two-ADUs-per-lot guide.
Best Two-ADU Combinations
Each combination has different cost, income, and lot requirements. Here are the four most common strategies, ranked by cost-effectiveness:
DADU + Basement Conversion
Best for: Homeowners with an existing full basement and sufficient backyard space.
The most cost-efficient two-ADU strategy if you have a suitable basement. The DADU commands premium rent as a standalone unit, while the basement conversion leverages existing space at a fraction of new construction cost. The basement typically takes 3-6 months; the DADU takes 10-12 months. Build them simultaneously to save on mobilization costs, or phase the basement first to generate income while the DADU is under construction.
Total Investment
$300,000 - $650,000
Monthly Income
$3,000 - $4,200
Annual Gross
$36,000 - $50,400
DADU + Garage Conversion
Best for: Homeowners with a garage they no longer need for vehicles.
Similar to the basement combo, but converts a garage instead. If you have a detached garage, the converted unit offers near-DADU privacy at conversion pricing. An attached garage conversion costs less but shares a wall with the main home. The garage conversion takes 3-6 months; the DADU takes 10-12 months. You lose vehicle storage, so plan for off-street parking.
Total Investment
$320,000 - $650,000
Monthly Income
$2,800 - $3,800
Annual Gross
$33,600 - $45,600
DADU + Attached ADU
Best for: Homeowners seeking maximum rental income who have a medium to large lot.
The highest income potential because both units are purpose-built with optimal layouts and finishes. The DADU provides maximum tenant privacy in the backyard; the attached ADU adds a second unit while sharing a wall and potentially some utilities with the main home. This combination works on medium to large lots where there is room for a detached structure plus an addition to the main home.
Total Investment
$350,000 - $800,000
Monthly Income
$3,200 - $4,400
Annual Gross
$38,400 - $52,800
Two Detached ADUs
Best for: Larger lots (7,500+ sq ft) with sufficient setback clearance for two structures.
Two standalone backyard cottages offer the most privacy for tenants and the highest per-unit rents. However, this combination requires the most lot space (room for two separate structures with setbacks from property lines and from each other) and the highest investment. Both units need separate utility connections. Best suited for larger lots in neighborhoods like Birchwood, York, or county properties with more acreage.
Total Investment
$400,000 - $900,000
Monthly Income
$3,400 - $4,400
Annual Gross
$40,800 - $52,800
All income projections are based on current Bellingham rental market data. Actual income depends on unit size, finishes, location, and market conditions. See our rental income guide for neighborhood-specific data.
Which Lots Support Two ADUs?
Not every lot can physically accommodate two ADUs. The key constraints are:
Lot Coverage
All structures on the lot — main home, ADUs, garage, shed — must stay within the maximum lot coverage for your zone. In most Bellingham residential zones, this is 35-50% of the lot. A 6,000 sq ft lot with 35% coverage allows 2,100 sq ft of structures total. If the main home is 1,500 sq ft, you have 600 sq ft of coverage remaining — enough for one ADU but tight for two. Conversions (basement, garage) do not add to lot coverage since the structure already exists.
Setback Requirements
Detached ADUs require setbacks from property lines (typically 5 feet in Bellingham). If you are building two detached structures, each needs its own setback clearance, and there may be separation requirements between structures. This means larger lots support two detached ADUs more easily. Attached ADUs and conversions avoid this issue since they are part of or within existing structures.
Utility Capacity
Two additional dwelling units require sufficient water, sewer, and electrical capacity. Most Bellingham lots with city water and sewer can support this, but older homes with undersized service lines may need upgrades. We assess utility capacity during the feasibility study and include any upgrade costs in our projections.
Lake Whatcom Watershed
Properties in the Lake Whatcom watershed face strict impervious surface limits that make two ADUs very challenging. The watershed's stormwater regulations limit total impervious surface, and two new structures may exceed the allowable amount. Watershed details.
The Phased Approach: Build One, Then Two
You do not have to build both ADUs at once. In fact, a phased approach is often the smarter strategy:
Phase 1: Build the First ADU
Start with the ADU that makes the most financial sense for your situation. If you have a suitable basement or garage, the conversion is often the best first move because it costs less and generates income faster. If new construction makes more sense, a DADU provides the highest income potential.
Phase 2: Collect Rental Income
Rent the first ADU and let the income accumulate. At $1,500-$2,200/month, you are generating $18,000-$26,400/year. This cash flow can fund or help finance the second ADU. Some homeowners use the rental income to make extra payments on a HELOC, building equity faster.
Phase 3: Build the Second ADU
With rental income from the first unit and the property value increase it created, you have more equity and cash flow to finance the second ADU. The first ADU's success also gives you confidence, experience, and a track record that makes lenders more comfortable with the second project.
Result: Two Income Streams
With both ADUs complete and rented, you are generating $33,000-$53,000/year in gross rental income from property you already owned. Your property value has increased by $150,000-$300,000+. And you have created a durable, inflation-hedged income stream that will continue for decades.
Permitting Two ADUs
The permitting process for two ADUs is straightforward in Bellingham:
Simultaneous Applications
You can submit permit applications for both ADUs at the same time. Each goes through its own Type I (administrative) review. This approach saves time and can save on design costs if the architect or designer plans both units together. Combined site plans also make it easier to optimize placement, utility routing, and construction sequencing.
Sequential Applications
You can also permit one ADU now and the second later. This works well with the phased construction approach. The second permit application is not affected by the first — each ADU is reviewed independently. Just make sure the first ADU's placement does not prevent the second one from meeting setback or coverage requirements.
Both ADUs follow the same Bellingham ADU permitting process: Type I administrative review, no public hearing, no discretionary denial. The city cannot prevent you from building two ADUs if your lot meets the physical requirements. See our permit timeline guide for expected review periods.
Neighborhoods Best for Multi-ADU Development
Two-ADU strategies work best on larger lots in neighborhoods with strong rental demand. These Bellingham areas are particularly well-suited:
Birchwood
Large lots, many with existing detached garages suitable for conversion. Room for a DADU in the backyard plus a garage conversion or basement unit. Close to downtown with strong rental demand.
York / Lettered Streets
Established neighborhood with larger pre-war lots. Many homes have full basements ideal for conversion. Central location near WWU and downtown drives premium rents. Strong demand from students, faculty, and young professionals.
Edgemoor
Generous lot sizes with established landscaping. Room for detached structures with adequate setbacks. Desirable waterfront-adjacent neighborhood commands top rents.
Whatcom County Properties
Rural and semi-rural lots with acreage offer the most space for two detached ADUs. County zoning allows ADUs on residential and agricultural-zoned parcels. Longer utility runs may add cost but lot constraints are minimal. County regulations.
Can Your Lot Support Two ADUs?
Our free feasibility study analyzes your lot coverage, setbacks, and utility capacity to determine exactly which two-ADU combinations are possible — and projects the income each combination would generate.
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Frequently Asked Questions
Can I really build two ADUs on one lot in Bellingham?
Yes. Washington's HB 1337 requires cities like Bellingham to allow at least two ADUs per residential lot. Bellingham's municipal code (BMC 20.10.036) permits any combination of types: two detached, one detached plus one attached, one detached plus one conversion, or other pairings. The constraints are lot coverage (all structures combined cannot exceed the maximum for your zone), setbacks, and utility capacity — not the number of ADUs. Our feasibility study calculates exactly what combinations your specific lot can support.
Do I need to build both ADUs at the same time?
No. You can build one ADU now and add the second later. Many homeowners take a phased approach: build the first ADU, start collecting rental income, and use that cash flow to fund or finance the second unit. You can apply for permits simultaneously or sequentially — each ADU goes through its own Type I (administrative) review. The phased approach also lets you learn from the first project and refine your approach for the second.
What is the best combination of two ADUs for maximum income?
For maximum rental income, DADU + Attached ADU is the strongest combination, generating $3,200-$4,400/month combined in Bellingham. However, the best combination for your specific situation depends on your lot, your budget, and your goals. If you already have a basement or garage suitable for conversion, converting the existing space ($100K-$200K) plus building a new DADU ($200K-$450K) may deliver the best return on investment because the conversion leverages existing structure at a lower cost.
Does having two ADUs require two separate utility connections?
It depends on the ADU types. A detached ADU typically requires its own water, sewer, and electrical connections. An attached ADU or conversion can often share utility connections with the main home (though a separate electrical panel is usually required). When building two ADUs, at least one separate utility lateral is almost always needed. The total utility connection cost for two ADUs ranges from $15,000 to $40,000, depending on the distance from the street and the types of connections required. We assess utility capacity during our feasibility study.
Double Your Property's Potential
Two ADUs can generate $3,000-$4,400/month in rental income from property you already own. Let us show you what combinations work on your lot.
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