Home additions are exciting — a new office, an expanded bedroom, a finished basement, or that long-awaited sunroom. But the moment homeowners start planning, the same HVAC question pops up:
“Do I need a bigger system?”
Most believe the answer is automatically “yes.”
After all, more square footage = more BTUs, right?
Not quite.
Adding living space does not always mean you need a new AC, a new furnace, or a completely new HVAC system.
What you do need is a recalculated load — a real understanding of your home’s new heating and cooling requirements.
The truth is:
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Your current system might already have spare capacity.
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A few duct adjustments could be all you need.
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A mini split might handle just the new space.
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Basement finishes often add less load than homeowners think.
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Over-sizing can actually make comfort much worse.
And when done right, additions increase comfort without inflating energy bills or overworking your equipment.
This guide walks you through exactly how to recalculate your tonnage, how additions impact real BTU load, and how to expand your living space without replacing everything.
Let’s start with the biggest misconception.
📏 1. You Don’t Size HVAC Based on Square Footage Alone — Especially After an Addition
Square footage is only one part of the story.
The U.S. Department of Energy explains that HVAC sizing must account for dozens of factors — windows, insulation, ceiling height, infiltration, climate zone, and building materials.
🔗 https://www.energy.gov/energysaver/central-air-conditioning
Additions complicate this even more because:
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Newer rooms often have different insulation levels
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Sunrooms have more windows
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Bonus rooms have hot structural locations
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Basements sit underground and stay cool
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Subfloor and attic transitions make airflow tricky
This means:
Two homes with identical additions can require two entirely different HVAC sizes.
A 250 sq. ft. basement rec room adds minimal load.
A 250 sq. ft. sunroom can add the load of an entire bedroom plus the heat of a greenhouse.
This is why recalculating tonnage is essential — not guessing.
📐 2. First Step: Understand Your Current System’s Actual Capacity
Before adding more load, you must determine:
Is your current system truly meeting your current home’s needs?
You need to look at:
✔ Blower performance
✔ Delivered airflow (CFM)
✔ Static pressure
✔ Whether the system short cycles
✔ Whether it runs constantly
✔ Whether certain rooms are already uncomfortable
✔ Whether humidity is too high
✔ Whether ductwork supports full tonnage
ENERGY STAR warns that duct issues alone can reduce capacity by 20–30%.
A “4-ton” AC may be delivering only 3 tons because of:
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Undersized return air
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Long duct runs
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Leaky trunk lines
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Restrictive filters
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High static pressure
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Poor blower settings
If your system is already struggling, an addition will push it over the edge.
If it’s running strong with spare capacity, you may not need to upsize at all.
📊 3. Second Step: Calculate the New Load From the Addition or Basement
This is where homeowners make the most assumptions — and the most mistakes.
Every addition increases load differently.
Here’s how major project types compare:
🏡 A. Finishing a Basement
Basements add surprisingly little cooling load because they’re surrounded by soil.
Advantages:
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Soil temperature stays cool year-round
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Little solar heat gain
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No direct sunlight
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High thermal mass
But they add moisture load — which impacts humidity, not tonnage.
BTU requirement for a basement is often 50–70% less than above-ground space.
🌞 B. Adding a Sunroom or Enclosing a Porch
This is the hardest type of addition to cool.
High solar gain increases BTU load exponentially.
If it has:
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Floor-to-ceiling windows
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South or west exposure
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Minimal insulation
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A vaulted ceiling
…it will dramatically raise cooling demand.
These spaces often need:
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A dedicated mini split
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Reflective window film
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Separate ducting
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Larger returns
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Solar shades
Trying to shoehorn them into the main HVAC system is usually a mistake.
🛏 C. Adding a Bedroom, Office, or Bonus Room
Here, the load depends on:
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Window direction
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Insulation
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Duct routing
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Second-floor vs main floor
Bonus rooms over garages are particularly challenging because garages are:
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Non-insulated
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Hot
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Exposed on multiple sides
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Poorly sealed
They often require more BTUs than their square footage suggests.
🔼 D. Expanding the Primary Suite
Large suites add load because:
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They often have oversized windows
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They may have direct afternoon sun
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Bathrooms add humidity
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They’re sometimes attached to attic knee walls
Depending on layout, you may need:
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Additional return air
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A zoning panel
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A dedicated duct run
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Or a half-ton capacity adjustment
🧱 E. Attic Conversions
Attics get brutally hot — especially in summer.
The Building Science Corporation highlights that attic temps can exceed 130°F on hot days.
🔗 https://buildingscience.com/
Even with insulation upgrades, conversions often require:
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Thick duct insulation
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Dedicated returns
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A mini split
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Extra BTU capacity
💨 4. Third Step: Evaluate Whether Your Ductwork Can Handle the Extra Airflow
You can have the perfect tonnage…
…but if the ductwork can’t move the required air, sizing is useless.
Your duct system must handle:
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New supply airflow
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New return airflow
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Balanced pressure
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Increased CFM without noise or resistance
The U.S. DOE warns that duct design dramatically affects system capacity.
Additions often require:
✔ Additional supply runs
✔ Additional return ducts
✔ Larger trunk ducting
✔ Booster fans
✔ Static pressure correction
If static pressure is already high, adding another branch run may bring the entire system to a crawl.
This is why recalculating tonnage involves ductwork — not just equipment.
🧮 5. Fourth Step: Determine Whether Your Current System Has Spare BTU Capacity
Not every system is running at 100% load.
Some homeowners have:
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Oversized systems
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Upgraded insulation
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Low solar gain
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Light occupancy
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Cool basements
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Efficient windows
In these homes, the existing system may already have extra capacity to support an addition.
To determine spare capacity, we look at:
✔ Runtime during hottest days
If it never runs more than 60–70% of each hour, you may have extra BTUs available.
✔ Temperature recovery
If your home cools quickly after setbacks, it may have spare tonnage.
✔ Humidity control
If humidity stays under 55%, your system isn't overworked.
✔ Static pressure
If pressure is low, airflow can be increased without stress.
✔ Supply temperature split
Correct split means the system is not struggling.
✔ Climate zone
Homes in milder climates need less tonnage.
This tells us whether you must upsize — or whether your current system can safely support the new space.
📉 6. When You Don’t Need to Replace the Entire HVAC System
This surprises most homeowners.
You may only need minor upgrades if:
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Your addition is under 300 sq. ft.
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Your basement is under 1,000 sq. ft.
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You’re adding a small office or bedroom
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Your home has spray foam insulation
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Your ducts have remaining capacity
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Your system is under 10 years old
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Your system rarely runs at full tilt
In these cases, you may only need:
✔ 1–2 new supply vents
✔ A new return grille
✔ A duct enlargement
✔ A blower-speed change
✔ A zoning panel
✔ A modest dehumidifier
✔ Better insulation or window film
These solutions cost a fraction of a new HVAC system.
And they work beautifully when sized properly.
🌀 7. When a Mini Split Is the Perfect Add-On for an Addition
Mini splits are incredibly efficient, inexpensive to run, and perfect for additions.
They work well when:
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Your main system is already properly sized
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The addition has unique heat load (sunroom, garage room, attic)
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Ductwork cannot be extended easily
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You want independent temperature control
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You want to avoid replacing the main HVAC system
Advantages:
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No ductwork needed
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SEER2 efficiencies often 20–30+
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Perfect humidity control
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Extremely quiet
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Temperature control by room
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Can heat + cool independently
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Great for offices, sunrooms, attics, bonus rooms
ENERGY.gov recommends ductless options for additions where ducts are difficult or expensive.
🔗 https://www.energy.gov/energysaver/ductless-mini-split-heat-pumps
📦 8. When You Do Need to Upsize the Main HVAC System
Larger system upgrades are recommended when:
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The addition increases total home size by 20–25%+
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The current system already runs long cycles
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You live in a hot-humid or hot-dry climate
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The addition adds major window area
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You’re building an oversized primary suite
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You’re adding major solar gain
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You’re converting attic space
In these situations, you may need:
✔ a half-ton increase
or
✔ a full-ton increase
or
✔ a two-stage or variable-speed system for high load swing
But even then, ductwork must be matched to the larger system.
🔧 9. Samantha’s Room-by-Room Load Recalculation (Homeowner-Friendly Version)
Contractors use Manual J — the official industry method.
You can understand the concept in a simpler way.
Here’s my simplified recalculation approach:
STEP 1 — Calculate load of the new space
Evaluate:
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New square footage
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Insulation level
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Windows and direction
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Ceiling height
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Climate zone
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Sun exposure
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Number of occupants
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Whether it’s below or above grade
STEP 2 — Assess existing system performance
Ask:
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Does it short cycle?
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Does it run constantly?
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Does the upstairs get too hot?
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Does humidity rise on warm days?
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Does the system cool evenly?
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Does it meet the load on 95°F days?
If your system struggles now, adding load makes the issue worse.
STEP 3 — Evaluate existing ductwork
Check:
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Static pressure
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Return duct size
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Trunk sizing
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Ability to add new branches
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Temperature split
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Airflow strength
If ducts can’t handle more CFM, upsizing equipment won’t help until ducts are corrected.
STEP 4 — Choose the best strategy
Depending on findings:
✔ Keep existing system + add duct
✔ Keep existing system + add return
✔ Use zoning
✔ Add a mini split
✔ Increase insulation
✔ Increase system tonnage
This is how you avoid over- or undersizing the entire home.
🧊 10. Special Case: Climate Zone Impact on Recalculated Tonnage
Your region dramatically affects HVAC sizing after an addition.
DOE climate zones:
🔗 https://www.energycodes.gov/determinations
Hot-Humid (FL, LA, GA, TX Gulf)
High humidity increases BTU needs.
Sunrooms and garage rooms are difficult to cool.
Mini splits are common.
Hot-Dry (AZ, NV, Inland CA, NM)
Solar gain raises BTU load heavily.
Additions with large windows often need dedicated cooling.
Mixed-Humid (TN, NC, VA, KY)
Balanced heating/cooling loads.
Duct design matters more than tonnage.
Marine (WA, OR, Northern CA)
Additions may require little cooling.
Mini splits are often sufficient.
Cold-Dry (MN, WI, ND, SD)
Additions often need more heating capacity, not cooling.
Split systems or heat pumps with good low-temp performance are important.
Climate changes everything — including how additions impact your HVAC system.
📉 11. The Biggest Mistakes Homeowners Make with Additions
Here are the pitfalls I see constantly:
❌ Extending a duct without checking airflow
This steals air from the rest of the house.
❌ Adding supply but no return
Creates pressure imbalance and weak cooling.
❌ Assuming a sunroom = “just another room”
Its load is usually 2–3× higher.
❌ Thinking a basement needs a ton of cooling
Basements need dehumidification more than AC.
❌ Replacing the whole system unnecessarily
When a mini split or duct upgrade would work better.
❌ Ignoring the climate zone
Sizing rules change dramatically by region.
❌ Forgetting ductwork must match new tonnage
Upsizing equipment without upsizing ducts = airflow disaster.
Avoiding these mistakes saves thousands — and keeps comfort high.
🏡 12. Real-World Scenarios: What Homeowners Should Do
These examples are based on typical cases:
Scenario A — Finishing an 800 sq. ft. basement in Ohio
Recommended:
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Add small supply/return ducts
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Add a dehumidifier
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No tonnage increase needed
Scenario B — Adding a 300 sq. ft. bonus room over a garage in Tennessee
Recommended:
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Dedicated mini split
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Improve insulation under floor
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No system replacement
Scenario C — Adding 400 sq. ft. sunroom in Florida
Recommended:
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Dedicated mini split
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Low-E window film
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Additional shading
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Do not tie into main ducts
Scenario D — Expanding primary suite by 250 sq. ft. in Texas
Recommended:
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Add return
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Add supply duct
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Possible 0.5-ton increase if system is already near max
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Evaluate duct capacity
Scenario E — Adding a home office in Washington State
Recommended:
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Add a small duct
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No tonnage changes needed
✔ Final Takeaway from Samantha
If there’s one message I want every homeowner to hear, it’s this:
You can expand your home without replacing everything — you just need a proper load recalculation.
When you size based on:
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Actual home performance
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Climate zone
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New load
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Existing duct capacity
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Solar gain
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Humidity
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Comfort needs
…you get an HVAC system that works smarter, not bigger.
Additions don’t require panic.
They require math — and a strategy.
And when you size correctly, you’ll enjoy:
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Perfect comfort
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Lower energy bills
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Better humidity control
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Long equipment life
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A home that feels beautifully balanced
No unnecessary upsizing.
No wasted money.
Just smart, data-driven comfort.
Buy this on Amazon at: https://amzn.to/47usZUk
In the next topic we will know more about: Oversized vs. Undersized: Samantha’s Diagnostic Checklist to See Where Your Home Stands
