A first estimate for a backyard cottage can look like one large number, but it is really a list of scope decisions. The short answer is that useful DADU estimate inclusions name design, permits, site work, utilities, building systems, allowances, exclusions, tax and contingency. A DADU, or detached accessory dwelling unit, is a separate home on the same lot as a house. Two bids with similar totals can describe very different projects, so scope deserves a closer look than price.

Details below reflect official Seattle and Washington pages checked in October 2026. Confirm with city staff for your property.

Why Headline Totals Can Mislead

Lining up bids and picking the lowest figure is tempting. A total only means something when you know what it covers, and a smaller number may simply reflect a smaller scope.

One bid might include removing an old shed and restoring the lawn. Another might list both as owner tasks. The second total looks lower, yet the owner still pays for that work later. A fair comparison starts with matching scopes, and the numbers become easier to read once each document describes the same project.

Design, Engineering and Permit Work

Design shapes everything that follows, so a bid should say who handles it. Some builders include design services. Others expect you to bring finished plans.

• Design responsibility. Does the price include drawings, or does it assume plans already exist?
• Engineering. Are structural or geotechnical services part of the scope?
• Permit preparation. Who prepares the application and answers city correction notices?
• City fees. Are permit fees inside the total, or will the city bill you directly?

Each answer changes what the total really means. It also shows who carries the work if reviewers ask for revisions. The post on what Seattle DADU permit fees cover explains those charges in more detail.

Site Work and Building Systems

Site work is one of the most variable parts of any bid. It deserves its own line items rather than a vague lump sum.

A clear document describes clearing, excavation, grading and foundation work. It notes how crews will reach the work area and where materials will be staged. Restoring lawns, paths or fences after construction belongs here as well.

Building systems need the same level of detail. Plumbing, electrical, heating and ventilation should each appear. Utility connections deserve extra attention, since the distance and capacity of each line can vary. The post on utility connections for a backyard cottage covers that topic.

How Allowances Fit In

Not every choice is final when a builder writes a bid. Many builders use allowances for items that are still open.

An allowance sets aside an amount for a category such as flooring, tile, lighting or appliances. Picking pricier items raises the budget. A cheaper one may lower it, and the contract usually explains what happens to the difference.

Ask whether each amount fits the quality level you expect. A low allowance can make a bid look attractive at first and lead to upgrades later.

Exclusions Deserve Equal Attention

Exclusions list the work a bid does not cover. They often explain why two totals differ so much.

• Landscaping beyond basic restoration
• Window coverings, furniture or decor
• Unexpected conditions found during excavation
• Utility and agency charges billed directly to the owner
• Changes requested after signing

An exclusion is not a warning sign on its own. It signals a cost to plan for separately. A document with no exclusions at all is more concerning, since it leaves too much open to interpretation.

Sales Tax and Contingency

Two more items often get overlooked. Sales tax comes first. The Washington Department of Revenue construction industry guide explains how retail sales tax applies to construction work for property owners. Ask each builder whether tax appears inside the total or as a separate line.

The second item is a contingency, a reserve for costs no one can predict, such as hidden soil problems. Some bids include one. Others expect the owner to hold it. The article on why a DADU budget needs a contingency explains how that reserve works.

A Hypothetical Side by Side

Imagine two example bids for the same one story cottage, invented here for illustration. Bid A carries a higher total. It includes design, permit preparation, shed removal, a new side sewer connection and lawn restoration.

Bid B shows a lower figure, and its fine print lists design, permit fees, the sewer connection and restoration as exclusions, with modest finish allowances. After you price those missing pieces and add them back, the gap may shrink or even flip. Neither bid is dishonest. They simply describe different projects.

Common Surprises in Estimate Review

• Permit fees and county sewer capacity charges sitting outside the builder total.
• Allowances set well below the finishes shown in sample photos.
• Site restoration limited to rough grading.
• Electrical service upgrades left as owner responsibility.
• No line for engineering on a sloped or wet lot.

Frequently Asked Questions

Should permit fees be inside the builder total?

Either approach can work if the document says which one applies. Some builders pay city fees and pass them through, while others leave payment to the owner. SDCI publishes a yearly fee subtitle and fee estimator, though the final amount depends on review. Ask each bidder to state how fees are handled so the totals line up.

What is a reasonable allowance amount?

There is no single figure that fits every project. A reasonable allowance matches the finishes you actually expect to choose. Bring photos or product examples to early meetings and ask the builder to price allowances against them. That makes each number easier to test and lowers the chance of upgrade costs appearing midway through the job.

How many bids should I compare?

No rule sets a number, and more documents are not always more useful. What matters is that each bid covers the same scope. Sending every builder the same plan set, site notes and finish expectations makes comparison far easier. A short list of complete, matching documents tells you more than a long list of vague ones.

Questions to Ask Every Bidder

• What design and engineering work is included?
• Which site and utility tasks are covered, and which are left out?
• What allowances are listed, and what quality level do they assume?
• Are sales tax and permit fees part of the total?
• How does the document handle unknown conditions?

These questions cover the main DADU estimate inclusions in plain terms. More planning topics are collected in the costs and budgeting section.