Evaluate Existing Utilities
Your electrical panel, HVAC system, and plumbing weren't designed for extra square footage. Before designing your addition, find out if these systems can handle the load—or if upgrades need to be in your budget.
Quick Summary
Electrical panel upgrade
$2,000 - $4,000
HVAC upgrade/addition
$8,000 - $15,000
Plumbing main line
$1,500 - $5,000
Electrical Capacity
Most homes built before 1990 have 100-amp service. Modern homes typically have 200 amps. Your addition will add electrical load—the question is whether your panel can handle it.
Check Your Panel
- 1Find your main electrical panel (usually in garage, basement, or utility room)
- 2Look at the main breaker at the top—it shows your total amperage (100A, 150A, or 200A)
- 3Count available breaker slots—if your panel is full, you'll need a subpanel or upgrade
You're Probably Fine If:
- • 200-amp service
- • 6+ available breaker slots
- • Adding bedrooms/living space only
You Likely Need Upgrade If:
- • 100-amp service
- • Panel is full or nearly full
- • Adding kitchen, bath, or HVAC
- • Planning electric car charging
Pro tip: Hire a licensed electrician for a load calculation ($100-200). They'll tell you exactly what your panel can handle and what upgrades are needed.
HVAC Capacity
Your heating and cooling system was sized for your current square footage. Adding space means either your existing system needs extra capacity, or you'll need a second system or zone.
HVAC Options for Additions
Extend Existing System
If your system has 25%+ extra capacity and ductwork can reach the addition. Cost: $1,500-5,000 for ductwork extension.
Add Zoning to Existing System
Install dampers and a second thermostat to control addition separately. Cost: $1,500-3,500.
Mini-Split System
Independent heating/cooling for the addition. No ductwork needed. Cost: $3,000-8,000 for single or multi-head unit.
New Full System
If existing system is old or undersized, replace it with a larger unit. Cost: $8,000-15,000+.
Don't Guess
Have an HVAC contractor do a Manual J load calculation ($150-300). This tells you exactly what BTU capacity you need for the entire house including addition.
Plumbing Capacity
If your addition includes a bathroom or kitchen, you need to verify your water supply and drain/sewer lines can handle additional fixtures.
Water Supply
- • Check main line size: 3/4" is standard for most homes. 1" is better for large homes or multiple bathrooms.
- • Test water pressure: Should be 40-60 PSI. Below 40 may indicate undersized supply.
- • Consider water heater: Adding a full bath may require larger or second water heater ($1,000-3,000).
Drain/Sewer Lines
- • Main drain size: Most homes have 4" main drain—can handle multiple bathrooms.
- • Distance matters: Far additions may need drain pump ($500-2,000) if below sewer line.
- • Septic systems: Adding bedrooms may require septic expansion ($5,000-15,000)—check local requirements.
Septic system owners: Many jurisdictions limit the number of bedrooms based on septic capacity. Adding bedrooms may require a larger septic system or secondary treatment—potentially $10,000-30,000. Check with your health department before designing.
What to Do Now
Check your electrical panel
Note the amperage and available slots. Take a photo for your contractor.
Note your HVAC system info
Find the model number, age, and tonnage (listed on outdoor unit or furnace).
Get professional assessments
Budget $200-500 for electrician and HVAC contractor evaluations before finalizing plans.
Frequently Asked Questions
Can I upgrade utilities during construction?
Yes, and it's often easier. But know the costs upfront so your budget is accurate. Discovering you need a $4,000 panel upgrade mid-project can derail your finances.
How much extra capacity does my HVAC need?
Rough estimate: 1 ton of cooling per 400-600 sq ft (varies by climate and insulation). A 500 sq ft addition needs approximately 1 additional ton. Get a proper Manual J calculation for accuracy.
Ready for the Next Step?
The final feasibility check is determining if your existing structure can support the type of addition you're planning.