house
Sunroom vs three-season porch: what the bid is actually selling
Three-season glass vs a year-round room, what those jobs cost, and the questions to put on the contractor before you sign.
You want more light and a place to sit that is not the deck in October. The person in the driveway says “sunroom” and writes a number. That word covers three different buildings: a screened porch with better windows, a three-season room you close up when it freezes, and a four-season addition with real heat and a foundation that acts like the rest of the house.
This is a hire job. You are not pouring footings or tying a roof into existing shingles from a kit video. The useful work this week is knowing which room you are buying, what has to be on the bid, and which questions make two quotes comparable.
What is the difference, in the house?
A screened porch keeps bugs off. It does not keep weather off. If you already have a roof and a slab, enclosing it with screens is the cheap end.
A three-season sunroom (sometimes a Florida room) is enclosed with glass or polycarbonate. You use it in spring, summer, and fall. Insulation is light or missing. Heat is a space heater you drag in, or spillover from the house when you open the door. Building departments often treat it as unconditioned space, which is why the permit and the appraisal can look different from a bedroom.
A four-season sunroom is a room. Insulated walls and floor, better glass (double-pane, often low-E), a foundation that goes to frost depth, and heating and cooling that actually serve that floor plate. Codes treat it as living space when it is built that way. Your tax card and your HVAC load change.
Angi’s 2026 comparison is the short version: expect $10,000 to $40,000 for a three-season sunroom and $25,000 to $80,000 for a four-season room with year-round gear. Three-season rooms skip dedicated HVAC and often use single-pane glass. Four-season rooms want insulation, better glass, and their own heat and cooling.
Angi: 3-season vs 4-season sunroom
If the brochure says “sunroom” and the bid does not say three-season or four-season, you do not know which of those ranges you are in.
What does this cost?
HomeGuide’s sunroom cost guide is the size table most quotes get compared against. Average installed cost $100 to $350 per square foot. A three-season room $8,000 to $50,000 ($80 to $230 per square foot). A four-season room $20,000 to $80,000 ($200 to $400 per square foot). A prefab sun porch or kit $5,000 to $30,000. A glass solarium $30,000 to $150,000.
For a 12-by-12 (144 square feet), their rows are $11,500 to $33,000 three-season and $28,500 to $47,500 four-season. A 10-by-10 is $8,000 to $23,000 vs $20,000 to $35,000.
Cost to Renovate’s 2026 sunroom page sits higher on the four-season side: three-season $15,000 to $45,000, four-season with HVAC $45,000 to $80,000, custom glass $80,000 to $150,000+. They put about $8,000 to $12,000 of a 250-square-foot gap on the glass upgrade, $4,000 to $7,000 on HVAC (ducts or a mini-split), and the rest on insulation, vapor control, and a real foundation.
Cost to Renovate: sunroom addition cost 2026
Use HomeGuide to sanity-check a size. Use Angi when someone is mixing “sunroom” with “year-round.” If a four-season bid for 144 square feet comes in under $20,000, something is missing: frost footings, insulation, HVAC, or the permit.
Prefab kits look cheap until you add a slab that is actually level, electrical, flashing at the house wall, and the person who will warranty the leak. The kit is materials. The bid is the building.
Which room should I actually buy?
Buy three-season when you will use it from April through October, you already sit on that side of the house in good weather, and you do not need another legal bedroom. It is the right product for coffee and plants, not for a home office in January in a cold climate.
Buy four-season when the point is year-round living space: a sitting room you will heat, a place for a desk, or square footage you want the appraiser to count. Then you are in addition territory. The HVAC question is not optional. HomeLight’s 2026 roundup quotes a builder who sees clients skip whether the existing system can serve the new glass box, then pay to upsize later.
A single-zone mini-split is a common way to condition a four-season room without ripping the whole duct system. That is a second trade and a second permit conversation. If the sunroom bid says “HVAC by others,” get that number before you sign the glass.
Do not buy four-season language on three-season bones. Single-pane glass, no insulation in the floor, posts on a patio slab that was never a foundation: that room will be a greenhouse in July and a porch in January, whatever the heading on the invoice says.
What do I ask the contractor?
Take these to the first meeting. Write the answers on the quote, not in a text thread.
Three-season or four-season, in writing. Plus glass type (single, double, low-E), wall and floor insulation (an R-value, not “insulated”), and whether the room is conditioned living space on the permit.
Foundation. Existing patio slab, new slab on grade, piers, or frost-depth footings. Ask whether that slab was designed for a room or for a grill. A four-season room on patio pavers is the leak and the bounce.
How the roof ties into this house. Step flashing, ice-and-water, whose shingles get stripped, who warrants the valley. Roof leaks at the house wall are the classic sunroom failure.
HVAC. None, a heater you buy later, extend the existing ducts, or a dedicated mini-split. If they extend ducts, ask whether they ran a load (Manual J is the written heating and cooling calculation for this house) or guessed.
Electrical. Outlets, lighting, a ceiling fan, and whether the panel has space. Glass rooms get hot; a fan is not a luxury line.
Permits and inspections, in their name. Setbacks, HOA, and whether the room counts as lot coverage. You want their license on the permit, not yours.
What is excluded. Paint, flooring, screens vs glass on one wall, gutters, mosquito-proofing the door, demo of the old deck, matching siding.
Who fixes a leak in year two. Workmanship vs manufacturer glass vs the roofer who was “a buddy.”
Get two bids that answer the same list. If one is 40% cheaper, find the missing line. A kit plus “you handle the pad” is not the same job as a custom four-season room.
What goes wrong after you sign?
The room is a solar oven. South- and west-facing glass without low-E, shade, or a way to dump heat. Ask about glass coating and whether a mini-split or a real fan is in this price.
The floor is cold in March. Three-season on an uninsulated slab. Fine if you expected that. A fight if you thought you bought four-season.
The house wall leaks. Cheap tie-in, no ice barrier, or a roof pitch that dumps water into the joint. This is why the flashing detail belongs on the bid, not “we’ll match it in the field.”
The HVAC cannot keep up. Extra glass is a load. If the furnace already struggles at 10°F, another 150 square feet of window will not help. Price the mechanicals with the room.
The appraiser shrugs. Unconditioned space often does not count like a bedroom. If resale square footage is why you are doing this, ask the contractor (and then your assessor) whether this permit is living space.
The kit sits on a patio that moves. Prefab walls on a slab that was never thickened at the edges. The doors stop latching. That is a foundation problem sold as a window problem.
How do I decide this week?
- Write down which months you actually need the room, and whether anyone will work or sleep in it.
- Pick three-season or four-season first, then collect bids. Do not let “sunroom” stand in for that choice.
- Ask every bid to name foundation, glass, insulation, HVAC, roof tie-in, permit holder, and exclusions.
- If the room needs heat and cooling, price that gear in the same week, including a mini-split if the ducts will not reach.
- Compare two quotes that answer the same list. If one will not write three-season vs four-season, you do not have a bid yet.
The cheap number is usually a different building. Decide which building you want, then hire the person who will put that building on paper.