Confidence: verified (standard US AEC practice, AIA phase structure) · Last verified: 2026-07-03 · Original SYMATEQ content.
US building design conventionally moves through Schematic Design (SD) → Design Development (DD) → Construction Documents (CD), following the AIA's standard phase structure — but what "complete" actually means for mechanical, electrical, and plumbing scope at each phase is easy to underestimate, because MEP deliverables mature at a different pace and shape than architectural deliverables do.
Schematic Design: MEP is shaping the building, not yet describing systems in detail
At SD, MEP scope is less about specific equipment and more about space and capacity claims that constrain the architecture — how much mechanical room and shaft space the systems will need, roughly where major equipment (chillers, switchgear, main distribution risers) has to sit, and preliminary load estimates (rough-order-of-magnitude HVAC tonnage, electrical service size) sized using simplified planning methods rather than final calculations (see ../engineering/mechanical/hvac-load-basics.md for exactly this kind of planning-grade estimate). This is the phase where a genuinely underestimated mechanical room or electrical service size gets baked into the building's floor plan and vertical circulation in a way that becomes expensive to correct later — SD-phase MEP input is disproportionately about protecting future flexibility, not about producing calculations a contractor could build from.
Design Development: systems get real, but drawings aren't yet permit-ready
DD is where preliminary single-line diagrams become actual one-line diagrams with real equipment selections, where duct and pipe routing gets laid out (not just sized), and where the engineer starts selecting actual equipment (a specific chiller model, a specific switchgear line) rather than a generic capacity placeholder. Load calculations mature from SD's planning-grade estimates toward the full method each trade's governing standard actually requires (full Manual J/N or ASHRAE load calculation for mechanical, full Article 220 calculation for electrical, full WSFU/DFU calculation for plumbing) — but DD drawings still generally aren't complete enough for permit submission or contractor pricing; they're the coordination set the architectural, structural, and MEP disciplines use to resolve conflicts (a duct routing through a beam, a panel location blocking a door swing) before committing to final construction documents.
Construction Documents: the set that actually gets permitted, bid, and built
CD is the complete drawing and specification package — full plans, schedules (panel schedules, equipment schedules, fixture schedules), one-line and riser diagrams, control sequences, and specifications — the version of the design that goes to the AHJ for permitting (see 02-permitting-utility-coordination-and-plan-review.md) and to contractors for pricing. Everything in this hub's calculator and code-reference content (Article 220 load calculations, WSFU/DFU sizing, duct sizing, ventilation rates) is aimed at producing CD-quality, permit-ready numbers — the level of rigor SD and DD deliberately don't require yet, because committing to that level of detail before the building's basic configuration is settled means redoing calculations every time the architectural plan shifts.
Why MEP coordination between disciplines matters more at DD than at any other single phase
DD is specifically where the three MEP trades' physical space claims collide most directly with each other and with structure — ductwork, cable tray, and piping all want the same ceiling cavity, and structural beam depths and locations constrain all three simultaneously. This is the phase where 3D coordination (clash detection between ductwork, conduit, piping, and structure) delivers the most value, precisely because DD-level routing is detailed enough to reveal real conflicts but not yet so finalized that resolving them requires redrawing a completed CD set.
Design-build and delegated design: a genuine variation on this sequence, not just a contractual label
Not every project follows the full SD/DD/CD sequence with the engineer of record producing every detail — design-build delivery and delegated design (where a specialty contractor, not the design engineer, produces final engineering for a defined system — fire sprinkler layout is the classic example, but this also shows up for some mechanical and electrical scope) shift some detailed design work later into the schedule, often into the construction phase itself as a "deferred submittal" (see 02-permitting-utility-coordination-and-plan-review.md). A design team working under one of these delivery models needs to be explicit, in the contract documents, about exactly which systems are delegated and what performance criteria the delegated design has to meet — an ambiguous delegation is a common source of coordination failure precisely because it isn't caught at DD the way a fully-engineer-of-record-designed system's conflicts would be.
Source / verify: AIA standard phase definitions (B101/A101 owner-architect and owner-contractor agreement structures), standard US MEP engineering practice for phase-appropriate deliverable maturity. Confirm the actual phase names, deliverable requirements, and delegated-design scope against the specific project's executed agreements — phase structure can be modified by contract.