ADU & New House Cost Calculator
The owners of this new home used a special spreadsheet containing secret formulas that calculated the cost of their new home within a fraction of the actual cost. It saved them thousands of dollars and prevented lost sleep. Copyright 2026 Tim Carter
ADU and New House Cost Calculator.
You may be sitting on the ADU or new-home-construction fence, wringing your hands and wallowing in worry about what it’s really going to cost. You’re frozen with fear that the price of your project will exceed your budget by many thousands of dollars. The risk is multiplied if you’re about to engage an architect to draw up plans for this undertaking. A custom plan in today’s market is going to set you back tens of thousands of dollars. Can you imagine the pain you’ll feel when a builder uses those plans and comes back with a price $100,000 or more higher than the budget you shared with your architect six months earlier? Don’t expect a refund from the architect.
Your inaction may cause even more pain. Inflation is raising the prices of both labor and materials for new homes. You might not think of something as simple as fuel. Diesel fuel prices are soaring. Hundreds, perhaps thousands, of gallons of this liquid gold will be burned directly and indirectly. Heavy equipment working on your job site and trucks delivering supplies burn diesel.
Imagine if you could conjure up a ballpark price of what your ADU or new home might cost using a very simple spreadsheet. I’m talking about one that contains built-in secret formulas used for years by me and just about every mortgage lender in the USA.
This simple tool empowers you to predict, with a high degree of accuracy, the actual construction cost. You just have to come up with some very important data you can share with a few subcontractors. This data can be located at most, if not all, websites that sell house plans. When you settle on a house that appeals to you, a large photo is displayed on its sales page, a small floor plan you can enlarge, and most importantly, the total square footage of the house is shown.
This free data will allow you to estimate the cost of quite a few things. The floor plan will show the layout of all the rooms. Window and door sizes can be estimated with a little help and common sense. All plumbing fixtures can be determined, even though the exact type of toilets, sinks, and faucets are not yet specified.
You can often pay a very reasonable fee to a subcontractor to provide you with an accurate estimate for the labor and materials. Do this with several different ones, and it will be a tiny fraction of what you would have paid an architect to produce full-blown plans.
A window and door supplier can use the information you gathered to produce an estimate for all your windows and doors. You’ll be asked for your input as to the level of quality you desire so they can give you an accurate estimate.
A roofer can look at the preliminary drawings and usually provide a very accurate price. You’ll just have to make a decision on the type of roofing you would use on your new home.
Armed with these fresh estimates and the spreadsheet I used for decades, you can now spend a few minutes and produce a very close price on what that house would cost to build in your city or town.
The ability of the spreadsheet to calculate the cost of the entire house based on just one estimate for say the roofing, plumbing, or electrical bids lies in a universal axiom adopted by mortgage lenders decades ago. Home construction lenders protect themselves as they disburse monies to the builder during the construction process.
The lender sends out an appraiser to the job site. This trained professional notes all those aspects of the house that are complete the day of his inspection. The lender doesn’t want to expose itself to a loss by giving out more money than is due at the time.
Years of lending experience, as well as access to detailed bids, allowed lenders to assign a percentage to each aspect of the home building process. For example, their research shows that the cost of installing a poured concrete foundation might represent X percent of the total cost of the house. The same is true for a list of 50-plus items on the spreadsheet.
A lending officer years ago shared a confidential appraisal worksheet with me that showed the percentages. I decided to check its accuracy decades ago against the detailed line-item bids I produced for homeowners. I was stunned by the accuracy of the secret percentages. This spreadsheet ended up saving me hundreds of hours of time. I could come up with a bid price by only getting bids from a few of my subcontractors. My total bid price was always very close to my competitors.
I’ve decided to make this spreadsheet available to you. It comes with a set of instructions detailing how to best use it to get a very accurate price for your ADU, your dream home, or a simple small home you’re thinking of building. The spreadsheet will work with any spreadsheet software.
I’ve lowered the price by a substantial amount as a gift to you. You can get your ADU and New House Cost Calculator here.
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Tim built custom homes and did ultra-custom remodeling for 20+ years before launching his Ask the Builder media presence. He was selected as one of the Big 50 by Remodeling magazine in 1993. This award gave birth to Ask the Builder in October 1993. Tim also operates https://DrawPlumbingPlans.com. His @askthebuilder YouTube channel was the first home improvement channel on YouTube.