$300,000 and up most likely.
For some folks, you may have seen that figure above and realized your dreams were crushed and decided to stop reading further. It might be cheaper outside of big cities, but nonetheless, the pandemic, supply shortages, and high demand, has rocked the construction world. Home values are up too, so the cost of construction is commensurate with the cost of buying/selling. If you’re still reading, then you are curious about why it will cost at least $300k, or maybe you already understand the cost and have the money available to do it.
At this time (updated in 2024), new construction in the Seattle area and many large cities is around $400 per sq ft if you’re really lucky, but usually more like $500 per sq ft and up for just basic construction quality (much different than the quality you see in design magazines). The stuff you see in magazines is often closer to $1000 per sq ft. If you are lucky, $400 per sq ft might get you a cookie cutter home, with hardie siding, basic trim, clunky ugly white windows, carpet, and drywall everywhere without any nicer finish materials.
For your typical addition, you are usually just adding onto a kitchen, or maybe adding a master suite. Let’s review a random 15’x20’ addition. That’s 300 sq ft. So, at $400 per sq ft, that is $120,000. That seems doable. You googled it, and the Google machine seems to say construction costs are even less than $400 per sq ft, so you may feel that $120k is a realistic construction budget. What the Google machine is not telling you about are the other factors that affect construction cost such as demolition, retrofitting existing conditions, economy of scale, permitting, and professional service fees (which we will explore below).
When you add onto a house, you inevitably need to tear into the existing house to make the connection between new and old. This costs something above and beyond the cost of building the new addition. The more you need to tear into, the more labor there is, the more temporary supports there are (to hold up the house while a wall is removed), and the more the dump fees there are. You could end up paying $30,000 or more in demo costs quite easily which makes that $120,000 addition go up to $150,000. When we design low budget additions, we try to impact the existing structure as little as possible to minimize these fees. Rather than expanding the entire back half of a house outward, it is more cost effective to make the addition “touch” the smallest part of the existing house as possible.
Retrofitting the existing house is one of the major expenses the happens with additions. When you finish the demo, all of the items that were removed will need to be rebuilt (to current code standards) and perfectly married into all the new addition work. This retrofitting of matching old and new is very painstaking labor since it must be done very surgically to protect the old parts from damage, and it simply takes more time build items that are not simple, repetitious tasks.
The more the new addition “touches” the old house, the more retrofitting there will be. Does the new roof impact the old roof? Will you need to re-roof all or part of the existing roof? If so, add at least $10,000 (or more when they discover rot and structural deficiencies in the existing roof). When the siding on the new addition is an extension of an old wall, all of that siding needs to be feathered together to blend seamlessly. It is often easier and cheaper to just remove the existing siding on that wall to just start fresh. Add another $10,000 or more for that to install new siding, trim, and paint on portions of the existing house that need to be redone. There’s a good chance some of those walls will be deficient in some way anyhow, so there will likely be additional scope to repair as needed.
Furthermore, older homes don’t meet the current codes for wind/earthquake or energy efficiency compliance, so once you touch part of the house, that part is now required to be brought up to current standards by filling it with more insulation and covering the walls with plywood and metal hardware to create “shear walls” (since older walls are usually covered in wood planks that don’t do anything to resist the side-to-side movement caused by wind and earthquakes). This will add another $10k or so for each area that needs retrofitted.
The same is true for flooring, ceilings, cabinetry, electrical, plumbing, heating/cooling systems, etc. Anything that gets affected by the demo will need to be updated to be able to tie into the new addition. Can the old furnace power the space of the new addition? Will you need to refinish most of the old floors to match the new ones that must tie in and match? Will the electrical wires need to be chased back to the panel and be updated to the current, safer Romex style? Will the foundation, that will now carry more weight, need to be reinforced (dig below it to add more concrete)? For any of the items above that pertain to your situation, add at least another $10,000 each. Let’s say our example 15’x20’ addition that is already up to $150,000 has 5 of these $10,000 retrofit items…. So, that is now $200,000 or more.
Economy of scale is another item that the Google machine doesn’t accurately portray when researching the cost of construction. When you are doing a small project, you will incur many of the same costs as a large project. For example, since you can’t drive a concrete truck into your back yard in many situations, a concrete pump truck is required. This will be about $1000 regardless of the size of the project. If the pump truck has to come to pump concrete into a 300 sq ft addition, or a 1000 sq ft addition, the cost is the same for the pump truck fee. For either size, you are paying for the cost of mobilization, 1 day of work, and demobilization. The same is true for many other trades like excavation, plumbing, electrical, roofing, siding, painting, scaffolding, and porta-potty rental. Those mobilization and rental fees cost the same regardless of the project size. So, the smaller project will cost more than $400 per sq ft. It may be more like $500 or $600 per square foot (or even more). That would bring the cost up to $250,000 or so for our example 15’x20’ addition. Add tax, and you’re closer to $275,000. Read our post called “Smaller Homes Cost More” if you’d like to learn more about economy of scale.
Permits are another expense. Depending on your situation, the permits might be straightforward, but usually there is a permit intake fee (a down payment for the building department to start reviewing your plans) which could be $2000 or $3000 for this small project (smaller cities tend to charge less). Once the building department finishes their first review, they may charge another fee. Another $1000 perhaps. They might do 2nd and 3rd reviews of the plans and charge another $1000 or $2000 to issue the permit. If there’s any steep slopes, shorelines, or other environmental critical areas, there could be another few thousand dollars for the review fees for the critical area permits. Plumbing, mechanical, and electrical all have their own permit fees too. There’s another $1000 or $2000. It’s not uncommon that permits could be $5000 or more, so now our example project is up to $280,000 or more.
Professional services are a major expense too. If you will be building your addition right up to your setback line, if you are maximizing the percentage of your property that is being covered by structures, or if the building department or architect requires a survey to be done for any myriad of other reasons, this could add another $3000-$6000 depending on the complexity of your property. If the soils need to be tested because they are in an area prone to settling, you may need a geotechnical report to test the strength of the soil and make suggestions for the types of foundations needed. That could cost $3000 or more.
You may need an infiltration test to determine how quickly (or slowly) the rainwater gets absorbed into your soil. That could be another $3000. If your addition is large enough, you may be required to design a system to capture the rainwater, so that it will slowly dissipate into the soil (instead of into the sewers). This civil engineering could cost another $5000.
You will need a structural engineer to design the foundation, shear walls, posts, beams, rafters, floors, nuts, bolts, etc for the addition. The structural engineer will also need to analyze the entire existing structure to determine how it will hold up (or be held up by) the new addition. This will add another $5000 or so.
If there’s any kind of water bodies or animal habitats, the building department might require you to hire an ecologist to create a mitigation plan to minimize impact to the environment. This could cost $5000.
If an archaeologist is required by the building department to monitor the excavations and to test the soil for any artifacts, that could add another $3000.
The architect needs to coordinate all of these engineers’ work into a cohesive set of documents that can be submitted to the building department to prove compliance. The architect also needs to measure the entire existing house, so the entire existing house can be accurately drawn which adds another $2000 or so. Obviously, the architect also designs and draws the proposed addition and creates a set of construction drawings and material specifications, so the builder has all the information needed to build the project (successfully). This is generally around 10% of the construction cost which would be around $28,000 in our example.
The total for professional services could be $30,000 to $55,000 depending on the complexity of the property - which could bring the cost of the example 15’x20’ addition over $300,000 (likely over $400k or $500k if it’s a complicated property with steep slopes or if you’re doing anything more than basic quality construction and finishes).
Once a couple more items get added to the scope (which happens in 100% of construction projects), the project will easily surpass $300,000. I hope this analysis has revealed the big picture of what to expect for any construction project. It is better to be well-informed before you start your project rather than to be fooled by salesperson gimmicks or wishful thinking. As architects experienced in construction, we are right with you. We think these numbers as freaking crazy, but this is what we are seeing over and over again from builders on our projects and others.’ To learn more about upper floor additions specifically, read our post here: additions
We have a handy dandy calculator that helps you (and us) estimate rough costs available at www.josharch.com/cost (it doesn’t work on mobile phones or tablets though)
If you’d like to learn more about our design process, visit www.josharch.com/process, and if you’d like to get us started on your project with a feasibility report, please visit www.josharch.com/help
