The Raspberry Pi HAT+ Standard Explained: Mechanical Spec, EEPROM ID, and Compatibility with Legacy HATs on Pi 5

If you've bought a HAT for a Raspberry Pi 5 and noticed the mounting holes don't quite match what you remember from a Pi 3 or Pi 4 project, you've run into the HAT+ specification — a mechanical and electrical update to the add-on board standard introduced alongside the Pi 5. This site's general HAT and pHAT ecosystem guide covers choosing and stacking boards day to day; this one focuses specifically on what HAT+ changes at the spec level and what that means for compatibility with your existing collection of legacy HATs.

Why the Standard Changed

The original HAT specification, dating back to the Raspberry Pi B+, required a board to align precisely with all four mounting holes on the Pi's footprint and to follow a fairly rigid mechanical template. That rigidity made sense for a smaller, simpler add-on ecosystem, but it became a real constraint as boards grew more varied — bigger add-ons with their own mounting needs, boards designed to stack with other boards, and enclosure designs that wanted more flexibility than "exactly four holes, exactly this size" allowed. HAT+ loosens the mechanical requirement specifically to accommodate that variety without losing the plug-and-play identification that made the original HAT standard useful in the first place.

What Actually Changed

AspectLegacy HATHAT+ Mounting holesMust align with all four standard mounting holesOnly required to mate with the 40-pin GPIO connector (including ID pins) and align with a single mounting hole Power statesNo standby state definedMust recognize a low-power state where the 3.3V rail powers down while 5V stays active, supporting more efficient standby behavior EEPROM identificationBasic board identification onlyExtended to support reporting on smaller, stackable HATs, opening the door to multiple compatible add-ons coexisting on one Pi Physical size flexibilityEffectively fixed footprint expectationsConsiderably more flexible board outlines, since only the connector and one mounting point are mandatory

Does This Break Your Existing HATs?

No — HAT+ is an additive specification for new boards, not a retroactive requirement that breaks older hardware. A legacy HAT built to the original four-hole specification still physically mounts on a Pi 5 (the Pi 5's GPIO header and mounting hole positions remain compatible with the legacy footprint) and still functions electrically, since the 40-pin GPIO interface itself hasn't changed. What you won't get from an old-spec HAT on a new Pi is the extended EEPROM behavior or the standby power handling that HAT+-compliant boards support; a legacy HAT simply behaves exactly as it always did, without those newer conveniences. The compatibility concern runs almost entirely in the other direction: a HAT+ board designed around the loosened one-mounting-hole rule may physically look and mount differently than what a Pi 4-era case or stacking bracket expects, so double-check physical fit before assuming a HAT+ board drops into an enclosure designed for the legacy footprint.

What This Means When You're Buying or Designing Boards

• When buying a new add-on board for a Pi 5 project, check whether it's labeled HAT+ or legacy HAT — both will work, but a HAT+ board is more likely to have been physically designed with the Pi 5's specific case and mounting ecosystem in mind.
• If you're designing your own add-on board (this site's KiCad and PCB design guides cover the electrical side), the loosened mechanical requirement under HAT+ gives you real layout freedom — you're no longer constrained to the historical four-hole footprint if your project doesn't need it, as long as you still mate correctly with the GPIO connector and provide at least the one required mounting point.
• For stacking multiple boards (a common want in sensor-heavy maker projects), check each board's documentation for explicit HAT+ stacking support rather than assuming any two add-on boards can share a header — electrical conflicts between boards using the same I2C addresses or GPIO pins are still possible regardless of which mechanical spec either board follows.

Checking What You Have

The official EEPROM utilities in raspberrypi/hats on GitHub let you read a connected board's identification EEPROM directly from Raspberry Pi OS, which will report the board's declared specification version along with its vendor and product information. Running this check against boards already in your parts bin is a quick way to confirm which spec a given HAT was actually built to before you commit it to a new Pi 5 project where standby power behavior or stacking might matter.

The Practical Takeaway

For most maker projects, HAT+ is invisible in day-to-day use — you plug a board in, it works, and the spec version underneath doesn't change how you write code against the GPIO or I2C bus. Where it actually matters is in physical planning: enclosure design, stacking multiple add-ons, and understanding why a shiny new Pi 5 HAT's mounting holes don't line up the way you remember from an older project. Keep that mechanical difference in mind when you're designing a case or a stacking bracket, and treat the EEPROM and standby-power improvements as a bonus you get for free on newer boards rather than something you need to actively manage.