Does every deck need a building permit?

No -- the 2021 International Residential Code exempts a deck only if it is not over 200 square feet, not more than 30 inches above grade at any point, not attached to the house and not serving the exit door the code requires. Towns often adopt the code with changes of their own, so ask the building department which rule applies to the house you're buying.

How high should the railings on a deck be?

Under IRC Section R312, a deck surface more than 30 inches above the ground needs guards on its open sides at least 36 inches high, or 34 inches along stairs, with no gap a 4-inch sphere could pass through. Measuring the height and testing the gaps on a walk-through tells you quickly whether the guards need work.

What should I ask the building department about an unpermitted deck?

Give them the address and ask whether any permit is on file for the deck, even an expired one; whether they would require an after-the-fact permit and what they would need to see; and whether there is an open code case on the property. An open permit that was never finalled is a different problem, with a different fix, from no permit at all.

Buying a House With an Unpermitted Deck? What to Check Before Closing

Q: Buying a House With an Unpermitted Deck? What to Check Before Closing

First check whether it needed a permit: the model code exempts small, low, detached decks. If it did, look at the guards, the stairs and how the deck is attached to the house, ask the building department what it would take to legalize, and get that priced as a credit, a repair or a holdback before you close.

Finding out a deck was never permitted isn't, on its own, a verdict on the deck. What matters before closing comes down to three things:

whether the deck needed a permit at all;
whether it is safe to stand on;
what fixing the paperwork would cost, and who pays.

All three can be answered inside a normal inspection period, if you start early.

First: did it even need a permit?

Not every deck does. The 2021 International Residential Code, the model code towns adopt, often with changes of their own, exempts a deck from a permit only if it meets all four of these conditions (Section R105.2):

not over 200 square feet;
not more than 30 inches above grade at any point;
not attached to the house;
not serving the exit door the code requires.

A small, low, free-standing platform in the garden may have been legal without paperwork. A raised deck bolted to the back of the house doesn't fit it. Your town may draw the line differently, so ask the building department which rule applies there. If the deck needed a permit and has none, keep going.

What you can check on the deck yourself

You can do this on a walk-through, without tools beyond a tape measure. It won't certify anything, but it tells you how worried to be.

Guards (railings). Under IRC Section R312, a deck surface more than 30 inches above the ground needs guards on its open sides. The guards must be at least 36 inches high (34 inches along stairs), with no gap a 4-inch sphere could pass through. Measure the height. Then try the gaps with something about 4 inches across. Low guards and wide gaps are easy to spot and easy to price.

How it's attached to the house. From underneath, find where the deck meets the house: the board bolted to the house is the ledger. Under IRC R507.9.1.1, a ledger may not hang on stone or brick veneer. Under R507.9.1.3, its fasteners must be hot-dipped galvanized or stainless steel. Look for rust streaks, soft or dark wood at the house wall, and whether there's metal flashing over the ledger. The code requires flashing where a deck attaches to a wood-frame wall (R703.4).

What it stands on. The code wants decks on concrete footings or another approved support (R507.3). Posts sitting on loose blocks, posts leaning, or a deck that moves when you shift your weight are reasons to bring in someone qualified.

Stairs. Note wobbly treads, uneven step heights or a missing handrail for your inspector.

How to legalize an unpermitted deck explains what a building inspector looks at once the boards come up, including the ledger connection, flashing and footing depth, in more detail than you can see from a walk-through.

Questions for the seller

Who built the deck, and when?
Was a permit ever applied for, or opened and never closed?
Has it been repaired, extended or re-boarded since?
Is it on the seller's disclosure?

The answers shape your next step, and they matter later too. If a problem surfaces after closing, what the seller knew and wrote down is the starting point. Can you make a seller pay for an unpermitted deck? explains why.

Make one call before your deadline

Call the building department for the town the house is in. Give them the address and ask three questions:

Is there any permit on file for the deck, even an expired one?
Would they require an after-the-fact permit, and what would they need to see?
Is there any open code case on the property?

An open permit that was never finalled is a different problem, with a different fix, from no permit at all. The answers turn "unknown cost" into something you can get quoted.

Settling it before you close

With the building department's answer and a contractor's or engineer's view in hand, you have four broad options:

A credit: the seller lowers the price and you handle the after-the-fact permit after closing.
A seller repair: the seller fixes the safety items, or legalizes the deck, before closing. Get it in writing, with the permit sign-off as proof.
A holdback: money held at closing until the work is done and signed off.
Walking away: if the deck is unsafe, the cost is unknown, and the seller won't move before your deadline.

Whichever you choose, get the deck's status and any agreed fix written into the contract or an addendum, not just an email.