The short answer: almost certainly yes, but which permit, and how much scrutiny it gets, depends on how the deck is built and which municipality you're in.
The rule that actually triggers a permit
Under Pennsylvania's Uniform Construction Code, a deck needs a building permit if either of two things is true: it has a roof, or any point of the deck surface sits more than 30 inches above the ground below it. A low, uncovered deck, one that never exceeds that 30-inch mark anywhere along its perimeter, can fall outside the building permit requirement. Plenty of Delaware County municipalities still want to know about it anyway, through a zoning permit, which brings us to the second thing worth understanding before you assume you're in the clear.
Building permit and zoning permit are not the same approval
A building permit is about how the deck is built: footings, framing, guardrails, the structural side, inspected against the Uniform Construction Code. A zoning permit is about where it sits on your property: how close to the property line, how much of your lot it covers, whether it fits your district's rules. A deck under 30 inches might skip the building permit and still need the zoning permit, because setback and lot-coverage rules don't care how tall the deck is. The two approvals run on separate tracks, and a municipality can require one without the other. This is the single most common reason someone assumes they're exempt and finds out otherwise.
What the building permit actually inspects
If your deck does need a building permit, here's roughly what an inspector is checking, so the process doesn't feel like a black box:
Footings have to reach below the frost line, typically around 36 inches deep in this part of Pennsylvania, though your specific municipality's building department sets and confirms the exact figure. This is what keeps the deck from heaving and shifting after a few winters of freeze and thaw.
Any point of the deck 30 inches or higher above grade needs a guardrail, generally a minimum of 36 inches tall, with baluster spacing tight enough that a 4-inch sphere can't pass through. This applies whether the deck attaches to the house or stands freestanding.
An attached deck, one that connects to the house rather than standing on its own posts, has its own set of structural requirements around how the ledger board fastens to the house and how the deck resists pulling away from it. This is a detail worth raising directly with your contractor or your municipality's building department if you're attaching to the house, since it's a common point of failure on decks that skip a permit and the inspection that comes with it.
Why the low-deck exemption is narrower than it sounds
Even where the county's Uniform Construction Code doesn't require a building permit, don't assume "no permit" means "no rules." Setbacks from the property line, how much of your lot is covered by structures, and whether an accessory structure like a deck is even allowed in your zoning district are all zoning questions, not building code questions, and they apply regardless of deck height. A municipality's zoning officer, not its building inspector, is who to ask about those.
What to actually do before you build
Call your municipality's code enforcement or zoning office before you buy lumber, not after. Ask two separate questions: does this need a building permit, and does it need a zoning permit. Bring rough dimensions and how high the deck will sit above grade. If a contractor is doing the work, most municipalities allow either the homeowner or the contractor to pull the permit, but confirm who's expected to do it in your specific town.
Skipping this step doesn't just risk a fine. It can mean tearing open finished decking for a late inspection, or having it surface as a problem when the home eventually sells and a buyer's inspector or lender flags unpermitted work.
More
Find your municipality's permit and zoning contacts. See also the permit FAQ, the glossary, and Do You Need a Permit for a Shed in Delaware County, PA?.