Cost of Central Air for a 900 Sq Ft House (Real-World Ranges + What Changes the Price)

If you’re pricing the cost of central air for a 900 sq ft house, I’m going to save you a bunch of time and frustration by saying the quiet part out loud:

There are two prices for central air.

1. The price when your ductwork already exists and is usable.

2. The price when we have to build the lungs of the house from scratch.
   

I learned this the hard way walking through a 900 sq ft place near downtown Lake Worth Beach. The homeowners weren’t asking me for the “best system.” They asked, “What’s the cheapest way to get central air?” And the answer depended almost entirely on what we found above the ceiling.

Here’s what those numbers tend to look like in the real world.

The quick answer: what most 900 sq ft central AC installs cost

For a small home, the equipment is not usually the scary part. The install conditions are.

Scenario A: existing, usable ductwork (simple change-out)

If you already have ductwork that is sized reasonably, sealed well enough, and laid out like it was meant for AC (not a science experiment), a straightforward change-out in a 900 sq ft home commonly lands around:

• About $7,000 to $9,000 for a clean swap in many small-home situations (equipment + labor + standard materials + permit handling)

On a national level, small-home replacement pricing can show up anywhere from roughly:

• About $5,000 to $13,000, depending on brand, efficiency tier, and what needs updating

This is the version of central air that feels “affordable.”

Scenario B: no ductwork (full conversion cost)

If you’re converting a window-unit house into true central air, the price jumps fast because you’re paying for the system and the airflow design.

In a typical 900 sq ft home, duct retrofit is often:

• About $30 to $50 per linear foot

• Many 900 sq ft homes end up needing roughly 60 to 90 linear feet, so ducts alone can be about $1,800 to $4,500
  

Then add the system, permit, and the wildcards (attic access, return air, drain routing, electrical adjustments, line set issues, patching and paint). When I’m being honest early, I tell people:

• About $10,500 to $14,500 all-in is a realistic “do it right” range for a 900 sq ft conversion with no real ducts
  

Here’s a quick way to visualize it:

What you’re paying for	With usable ducts (change-out)	No ducts (full conversion)
System install (equipment + core labor)	Often included in $7,000 to $9,000	Often mid $7,000s to low $10,000s depending on efficiency/details
Ductwork	$0 (or minor fixes)	About $1,800 to $4,500 (60 to 90 ft at $30 to $50/ft)
Permit (example market)	About $100 to $125 (often bundled)	About $100 to $125 (often bundled)
Common wildcards	line set, drain, electrical tweaks	return-air design, access time, patch/paint, electrical/line set

In that Lake Worth house, the moment we checked the attic access, the budget story changed. Tiny hatch, tight clearance, awkward runs, and no real duct system. Small house did not mean small job.

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What size AC do you need for a 900 sq ft house?

Most 900 sq ft homes land around a 1.5 ton system after a proper load calculation, but I never treat that like a universal truth.

A real sizing decision considers insulation, windows, sun exposure, ceiling height, infiltration, duct leakage, and how the house handles humidity.

Why many small homes land near 1.5 tons (and when they don’t)

A “ton” is just capacity. Roughly speaking, 1 ton equals 12,000 BTU/hr, so 1.5 tons is about 18,000 BTU/hr. That can be perfect for a lot of 900 sq ft homes, especially in hot climates, but plenty of 900 sq ft houses belong in other buckets:

• 1 ton: tighter envelope, shade, good insulation, modest glass, low ceilings, good ducts

• 1.5 ton: very common middle ground

• 2 ton: lots of sun, leaky house, poor insulation, high ceilings, big glass, or truly rough duct conditions
  

When that Lake Worth couple asked for the cheapest option, I told them something I repeat constantly:

I’m not upsizing the unit just to “make it colder.” If I oversize a small home, it short-cycles, and you end up cold but clammy. Comfort is temperature plus humidity, especially in Florida-style weather.

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The “small house trap”: oversizing, short-cycling, and humidity

Here’s the trap: people assume a small house should be cheap to cool, so they want to “go bigger to be safe.” But oversized systems can:

• Run in short bursts instead of long, steady cycles

• Pull less moisture out of the air

• Create uneven rooms because the ducts and returns are not designed for that airflow

• Wear components faster from frequent starts
  

That’s why I’d rather sell you the right-sized system with a sensible duct and return-air layout than a bigger tonnage number that looks impressive on paper.

Cost breakdown: where your quote actually goes

If two quotes for the same 900 sq ft house are thousands apart, it’s usually because something is different in the scope, not because one contractor is magically “half price.”

Here’s what I look for in a quote.

Equipment: condenser, coil/air handler, thermostat

In a small home, you’re typically buying:

• Outdoor unit (condenser or heat pump)

• Indoor coil and air handler (or coil paired with a furnace)

• Thermostat (basic or communicating)

• Matching system components that are meant to work together (this matters for efficiency and warranty)
  

If you’re shopping efficiency, this is where 17 SEER AC unit cost questions start. Just be careful with the phrase “unit cost.” People mean different things by it:

• Equipment-only cost (just the outdoor unit, or outdoor plus air handler)

• Total installed cost (equipment + labor + materials + permit + setup)
  

You want installed cost, because that is what you actually pay.

Labor + materials: line set, electrical, drain, pad, startup

Even with the same equipment, labor can swing hard based on access and complexity.

Common line items include:

• Refrigerant line set (or reuse if appropriate and clean, which is not always possible)

• Electrical whip and disconnect (and sometimes breaker or panel adjustments)

• Condensate drain routing, safety switch, overflow protection

• Pad, mounting, vibration isolation

• Startup and commissioning (refrigerant charge, temperature split, static pressure, airflow)
  

That Lake Worth attic hatch is a perfect example. Tight access and awkward runs mean more labor hours, more frustration, and more opportunity for shortcuts. I’d rather spend the time and do it right than deliver a cheap install that never feels even.

Permits and inspections

In my Palm Beach County area experience, I tell homeowners to budget roughly:

• About $100 to $125 per unit for a basic HVAC change-out permit
  

Some contractors bundle it into the quote, some list it separately. Either way, I want it included. No permit often means no accountability.

Ductwork and return air: the part that makes rooms feel “even”

If you want central air that feels consistent in every room, ductwork and return air are not optional details.

When that Lake Worth couple said, “We don’t want the cheapest. We want it to feel even,” that instantly told me we needed:

• A duct layout that balances rooms instead of blasting one hallway

• Return air that makes sense (not a tiny return choking the whole system)

• Airflow designed for comfort and humidity control, not just cold air output
  

This is also why “no ductwork” conversions can land in that $10,500 to $14,500 reality zone. You are buying comfort engineering, not just a metal box.

17 SEER AC unit cost: what changes at 17 SEER (and what you really get)

Efficiency is real, but it’s not magic. In a 900 sq ft house, the biggest “upgrade” you can buy is often a quality install with good airflow.

Still, I get why people search 17 SEER AC unit cost: you want to know what the upgrade costs and whether it is worth it.

Typical 17 SEER installed cost range (vs mid-tier)

For a small-home change-out, I generally think of 17 SEER as a step up that can raise the installed price compared with the baseline tier.

A practical way to look at it:

• Baseline efficiency tier change-out in a 900 sq ft house often sits around $7,000 to $9,000 in straightforward situations

• A 17 SEER class system can push that higher depending on features (and especially if it involves variable-speed components)
  

Instead of promising a universal number, here’s the honest truth: the “17 SEER premium” is quote-dependent. It is often driven by whether the system is:

• Single-stage vs two-stage vs variable-speed

• Basic thermostat vs communicating controls

• Standard install vs difficult access, line set work, electrical upgrades
  

If you want to price it cleanly, ask every contractor to quote two options with the same scope:

1. a baseline tier system, and

2. a 17 SEER class system,with model numbers listed.
   

Then you can see the real delta in your market.

When 17 SEER makes sense in hot, humid climates

In my Florida and Southeast type installs, I care less about the efficiency number by itself and more about what comes with it.

17 SEER class equipment often correlates with better comfort features, like:

• Longer run times at lower output (better moisture removal)

• Smoother temperature control

• Quieter operation
  

If your home runs the AC a lot, or you struggle with humidity, the comfort benefits can be worth more than the pure electricity math.

Variable-speed vs single-stage: comfort and moisture control

This is where small homes can win. A variable-speed style setup can run longer and steadier, which helps with humidity and room-to-room balance.

But I’ll say it bluntly: a high-SEER system installed on bad ducts is still going to feel bad. If your ductwork is undersized, leaky, or missing returns, you can spend more and still be unhappy.

The biggest factors that swing price on a 900 sq ft install

If you want to predict your quote before the contractor even shows up, focus on these.

Attic/crawl access and duct routing difficulty

This is the Lake Worth story in one sentence: the attic access can turn a “simple job” into a “long day.”

Low clearance, tight hatch, difficult runs, and awkward return-air paths increase labor, and labor is expensive.

If you have no ducts, routing can also trigger patching and paint. When ceilings have to be opened, you are paying for more than HVAC.

Electrical adjustments and “surprise” line set issues

Small homes are notorious for this:

• Older electrical panels

• Undersized breakers

• Questionable disconnect placement

• Line sets that are the wrong size, contaminated, or routed in a way that is not reusable
  

These are not “gotchas.” They are the hidden reasons two bids can look wildly different.

Brand, warranty, and installer quality

I’ve seen budget equipment run beautifully with a careful install, and I’ve seen premium equipment feel awful because the ductwork was ignored.

When comparing quotes, I care about:

• Clear scope

• Model numbers

• Permit included

• Ductwork and return-air plan (even if it is “existing,” it should be evaluated)

• Startup and airflow commissioning
  

If a quote is dramatically cheaper and vague, you’re often buying shortcuts.

How to get quotes you can actually compare

If you do one thing after reading this, do this: force every contractor to quote the same scope.

What to ask for: load calculation, duct design, return sizing

For a 900 sq ft house, these questions separate pros from salespeople:

• Did you do a load calculation (or at least document assumptions)?

• What tonnage are you proposing and why?

• Are you changing any ductwork, returns, or registers?

• How will you verify airflow and static pressure?

• What is included in the startup process?

• What permit is being pulled and who is responsible?

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I told that Lake Worth couple I would not oversize the system just to make it “feel colder,” and I meant it. A contractor who jumps to bigger tonnage without explaining load and airflow is not protecting you.

Quote checklist (copy/paste and use it)

Use this list to compare bids apples to apples:

•  Outdoor and indoor model numbers listed

•  Efficiency tier stated (SEER2 class) and any special features noted

• SEER2 minimum efficiency standards by region

•  Scope of ductwork work clearly defined (none, minor repairs, full retrofit)

•  Return-air plan described (size and location)

•  Line set plan stated (reuse or replace)

•  Electrical work included or excluded clearly

•  Condensate drain routing and safety switch included

•  Permit and inspection included

•  Startup and commissioning steps included

•  Warranty terms written down (parts and labor, and who honors it)
  

Red flags that predict comfort problems later

I watch for these:

• “We always put in a 2-ton for houses like this” with no load discussion

• No mention of return air

• Ductwork described as “as needed” with no dollar range

• No permit

• No plan for condensate safety (especially in humid climates)

• No commissioning (airflow and static pressure ignored.
  

FAQs

How much does central air cost for a 900 sq ft house?

If you have usable ductwork, many straightforward change-outs land around $7,000 to $9,000. If you have no ducts and need a true conversion, a realistic all-in range is often $10,500 to $14,500, plus any access, electrical, or repair wildcards.

What size AC do I need for a 900 sq ft home?

Many 900 sq ft homes land around 1.5 tons, but I size based on the actual heat load. Insulation, windows, ceiling height, sun exposure, and duct leakage can push it down or up.

Is a 17 SEER system worth it in a small house?

Sometimes. In a small home, comfort and humidity control often matter more than chasing the highest efficiency number. If the 17 SEER class option includes variable-speed comfort features, it can be worth considering, but only if your ductwork and return air support it.

What does “17 SEER AC unit cost” usually mean?

Homeowners use it two ways: equipment-only (just the unit) or total installed price. For budgeting, always ask for installed cost with model numbers and a defined scope, because installation details can swing the total dramatically.

How much does it cost to add ductwork to a house with no ducts?

A common ballpark is about $30 to $50 per linear foot. A 900 sq ft home often needs about 60 to 90 feet, so ducts alone can be about $1,800 to $4,500, not counting drywall or paint repairs if ceilings get opened.

What should be included in an HVAC quote?

At minimum: equipment model numbers, efficiency tier, permit handling, line set plan, drain plan, electrical scope, and some form of airflow and startup commissioning. If the contractor never talks about return air, I get concerned fast.

Conclusion

If you came here for the cost of central air for a 900 sq ft house, the most accurate answer is not a single number. It’s a fork in the road:

• Usable ducts already there: central air can be relatively straightforward, often around $7,000 to $9,000 in many small-home change-outs.

• No ducts: you’re building the airflow system plus installing the equipment, and that’s where quotes often become $10,500 to $14,500 all-in when done properly.
  

In my Lake Worth world, the homeowners who end up happiest are the ones who stop chasing “cheapest” and start chasing “even comfort in every room.” That usually means the right-sized system, a sensible duct and return design, and an installer who treats airflow and humidity like part of the job, not an afterthought.