A poorly designed group address (GA) structure is the single most common cause of KNX project maintenance nightmares. It adds hours to every service call, makes handover to other engineers nearly impossible, and causes subtle bugs when GAs are reused for conflicting purposes. Investing 30 minutes in GA structure design before touching ETS6 pays back on the very first service call.

The 3-level hierarchy explained

KNX ETS6 supports three GA formats: free, 2-level, and 3-level. Always use 3-level for any project with more than 20 devices. The 3-level format is X/Y/Z where: **X = main group (0–31)** represents the functional category (lighting, blinds, HVAC); **Y = middle group (0–7)** represents the physical zone (floor, apartment, wing); **Z = sub-group (0–255)** represents the individual device output or channel. Example: 0/2/5 = Lighting (0), Second floor (2), Bedroom 1 ceiling light ON/OFF (5). Any engineer opening ETS6 for the first time immediately understands what 0/2/5 does.

Main group conventions for EU projects

A consistent main group standard makes projects transferable between engineers. Recommended allocation: 0 = Lighting (switching, dimming). 1 = Blinds and shutters. 2 = HVAC (heating, cooling, ventilation). 3 = Security (alarm zones, motion, door contacts). 4 = Scenes. 5 = Status and feedback. 6 = AV and multiroom. 7 = Energy (metering, EV, solar). 8 = Access (intercoms, locks). 9 = Time and calendar. Reserve 10–15 for future expansion. Never mix functional categories in one main group — do not put "kitchen light" in main group 0 and "kitchen dimmer value" in main group 7.

Zone mapping example: 3-storey villa

Middle group maps to floors and areas. Floor 0 = ground floor (living, kitchen, entrance). Floor 1 = first floor (bedrooms, bathrooms). Floor 2 = second floor (study, guest room). Floor 3 = exterior (garden lighting, pool, carport). Floor 4 = technical (plant room, meter cabinet). For an apartment with multiple units: middle group = apartment number (0 = common areas, 1 = apartment 1, 2 = apartment 2). This makes the project structure self-documenting.

Sub-group example: living room lighting

0/0/0 = Living room ceiling light ON/OFF (DPT 1.001, command). 0/0/1 = Living room ceiling light DIM value (DPT 5.001, 0–100%). 0/0/2 = Living room ceiling light status feedback (DPT 1.001, read-only). 0/0/3 = Living room accent light ON/OFF. 0/0/4 = Living room accent light DIM value. 0/0/5 = Living room accent light status. 0/0/6 = Living room total scene (DPT 18.001, scene number). This pattern — command, dim value, status, repeat per circuit, then scene — is predictable and memorable.

Feedback GAs: the rule every project must follow

Every actuator output must have a dedicated status (feedback) GA. Never use the same GA for command and status. Using the same GA causes loop problems: the actuator sends its status, which another device interprets as a new command, which triggers the actuator again. The status GA (in main group 5 or as sub-group offset +2 from the command GA) receives the actuator's physical state and sends it to room displays, visualisation, and HA. Without feedback GAs, your KNX display shows "assumed" state rather than "actual" state — a critical reliability issue.

Scene GA structure

All scenes for a room go on a single GA using DPT 18.001 (8-bit scene number). Example: GA 4/0/0 = Living room scene. Value 1 = Standard (lights 80%, blinds auto). Value 2 = Cinema (lights 0%, blinds down, projector on). Value 3 = Dinner (lights 60%, warm white if tunable). Value 4 = Romantic (lights 20%, accent only). This is far cleaner than one GA per scene. The MDT and Gira pushbuttons support DPT 18.001 scene recall natively. In ETS6: configure each scene under the device's "Scene" communication object, not as separate GAs.

Common mistakes to avoid

Using 2-level GA structure in projects over 20 devices — the flat structure becomes unreadable immediately. Not allocating feedback GAs upfront — adding them later requires re-downloading dozens of devices. Mixing zone and function in the main group (e.g. main group 0 = ground floor, main group 1 = first floor) — this breaks when you add a second floor that also has lighting. Using sequential sub-group numbers without spacing (0, 1, 2, 3...) — when a circuit is added mid-project, there is no room to insert it logically; always leave gaps (0, 10, 20, 30...). Not documenting the GA structure in a separate spreadsheet — the ETS project file is not readable by the building owner or the maintenance electrician.

A clean structure pays back immediately

A well-designed GA structure means any certified KNX engineer can understand your project in 10 minutes without reading documentation. It reduces commissioning errors, simplifies troubleshooting, and makes handover professional. This is the difference between a project that clients recommend and one that generates complaint calls three years later.