Match Interior Floor Heights
One of the most frustrating surprises in any home addition: the new floor is 1-2 inches higher or lower than your existing floor. This common issue has solutions, but it's much easier to address before the subfloor goes down.
Quick Summary
Time needed
1-2 days
Cost
$500-$3,000
Professional help
Carpenter/framer
Why This Step Matters
Walking from your existing home into the addition should feel seamless—not like stepping up or down into another space. Floor height differences are tripping hazards, accessibility issues, and constant visual reminders that your addition was added later. The key is to identify and fix this issue before finishing materials go down.
Test BEFORE Subfloor
Use a long level and string line to check floor heights as soon as joists are complete—before subfloor is installed. Fixing the issue at the joist level is 10x easier than after subfloor and finished flooring are in place.
Why Floor Height Differences Happen
Several factors contribute to floor level mismatches:
Different Joist Sizes
Your existing home might have 2x8 joists, while modern code requires 2x10s for the same span. The 2-inch difference shows up in floor height. Similarly, switching from dimensional lumber to engineered I-joists can create height variations.
Foundation Height Variations
Even careful foundation work can result in slight height differences. A 1/2-inch difference in the foundation wall is common and often unnoticed until floors meet.
Different Subfloor Thicknesses
Older homes might have 5/8-inch subfloor while current standards use 3/4-inch. Some older homes have double subfloors. These differences add up.
Settling and Sagging
Older homes often settle unevenly. Your existing floor might not even be level itself, making it impossible to match a perfectly level new addition floor.
Different Finish Floor Thicknesses
If you're installing tile (thin) in the addition but matching hardwood (thicker) in the existing space, the subfloor heights need to account for different finish floor thicknesses.
How to Measure Floor Height Difference
Establish a Reference Point
In the existing house, near the transition point, mark a level line on the wall at a consistent height (like 36 inches above the finished floor).
Use a laser level: A self-leveling laser makes this much easier and more accurate than a traditional level and string.
Extend the Level to the Addition
Using the laser or a long level, extend that reference line into the addition space. Mark the wall in the addition at the same level.
Measure Down to Each Floor
Measure from the level line down to the subfloor (or top of joists if subfloor isn't installed yet) in both spaces. The difference between these measurements is your floor height discrepancy.
- Existing home: 36 inches to floor = baseline
- Addition: 37.5 inches to floor = 1.5 inches lower than existing
Account for Finish Floors
Add the thickness of your planned finish floors to each measurement. If the existing space has 3/4-inch hardwood and the addition will have 3/8-inch tile, you need to adjust.
Calculate final heights: Subfloor height + finish floor thickness = final floor height. Both spaces should have the same final height.
Solutions for Floor Height Differences
The best solution depends on how much difference you're dealing with and when you catch it:
Small Differences (Under 1/4 inch)
These can often be handled with transition strips or slight feathering of self-leveling compound. Not ideal, but usually acceptable.
Best fix: Beveled transition strip at doorway
Moderate Differences (1/4 to 3/4 inch)
The sweet spot for several fixes. Can be addressed with additional subfloor layers, furring strips on joists, or self-leveling compound.
- - Add 1/4" or 1/2" plywood layer to lower floor
- - Use thicker underlayment on lower floor
- - Fur out joists if before subfloor installation
Large Differences (3/4 to 2 inches)
Requires more significant intervention. Multiple subfloor layers become bouncy; furring is the preferred approach if still at joist stage.
- - Fur out joists with ripped lumber (before subfloor)
- - Sleeper system over existing subfloor
- - Lower new joists (adjust foundation)
- - Ramped transition over 4-6 feet (less ideal)
Major Differences (Over 2 inches)
This indicates a design or construction error that should have been caught earlier. Solutions are expensive and may require structural changes.
- - Rebuild foundation ledge at correct height
- - Create stepped floor with proper landing
- - Accept as a design feature with safety railings
Warning: If you discover a 2+ inch difference after framing is complete, this is a significant issue that needs to be discussed with your contractor and possibly your building inspector.
Detailed Fix Methods
Furring Out Joists
Best for: Before subfloor is installed
- - Rip lumber to exact needed thickness
- - Glue and screw to top of each joist
- - Use long straightedge to verify level
- - Cost: $200-$500 for materials
Sleeper System
Best for: When subfloor is already down
- - Install 2x4s flat on existing subfloor
- - Shim to achieve level
- - Install new subfloor on top
- - Cost: $500-$1,500 for typical room
Additional Subfloor Layer
Best for: Differences under 3/4 inch
- - Add 1/4", 1/2", or 3/4" plywood
- - Glue and screw to existing subfloor
- - Offset seams from layer below
- - Cost: $1-2 per sq ft materials
Threshold Transition
Best for: When flush floors aren't possible
- - Custom wood or metal threshold
- - Gradual ramp over 2-4 inches
- - Must meet ADA if accessibility needed
- - Cost: $50-$200 per doorway
How to Prevent Floor Height Issues
Include in design drawings
Architectural plans should specify finish floor heights relative to a benchmark
Measure existing floor stack-up
Before design, measure exactly: floor to subfloor to joist to foundation
Check after foundation pour
Verify foundation height matches plans before framing begins
Verify before subfloor installation
Final check at joist level—much easier to fix here than later
Communicate finish floor choices early
Your contractor needs to know if you're installing tile vs. hardwood vs. carpet
Frequently Asked Questions
What if my existing floor isn't level?
This is common in older homes. You have two choices: match the slope of the existing floor (maintains the "feel" but propagates the problem), or build the addition level and create a transition. Most designers recommend building level and handling the transition at doorways.
Can self-leveling compound fix large differences?
Self-leveling compound is designed for imperfections up to about 1 inch—not for correcting major height differences. It's also expensive at that thickness ($8-15 per square foot) and adds significant weight. Use structural solutions for differences over 1/2 inch.
Should I match my existing floors or use different flooring?
This affects your height calculations. If installing the same flooring, heights must match exactly. If using different flooring (e.g., existing hardwood, new tile), you can use the thickness difference as part of your leveling strategy. Plan this before framing.
Is a small step between floors acceptable?
Code typically allows up to a 7-inch step with a landing, but a small unexpected step (under 4 inches) is a tripping hazard and poor design. Either make it flush, or make the step obvious enough that people expect it. A 1-inch step is the worst—easy to miss, hard to fix.
Ready for the Next Step?
With your floors properly aligned, the next phase involves extending your home's electrical system into the addition—a critical step that requires careful planning to meet code requirements.