Project phases are time-bound sections of a project, each ending with a verifiable result and a milestone. The common 4 phase model splits a project into definition, planning, execution and closure. For German building projects, the fee regulation HOAI defines 9 work stages; the five process groups of the PMBOK Guide, by contrast, are not phases.
What Are Project Phases?
A project phase is a time-bound section of a project in which the team produces a defined result. It ends with a milestone: the client or steering committee reviews the result and releases the next phase. Because every phase has its own result, its start and end are easy to define.
Planning the project phases is one of the first management tasks in a project. Whether your project is large or small, phases make progress and quality verifiable at fixed points.
Some methods distinguish between project phases and project management phases. The latter are related management activities that you carry out several times within one project phase. PRINCE2, for example, distinguishes between technical stages and management stages.
Project phases come from traditional planning methods but also fit agile methods. In that case you combine linear phase planning with iterative work inside the phases.
What Is the Difference Between a Phase, a Process Group and the Life Cycle?
A phase is a section of time with its own result, a process group is a set of management processes, and the project life cycle is the sequence of all phases from start to closure. If you mix up the terms, you schedule monitoring as a separate block of time instead of an ongoing task.
| Term | What it describes | Example |
|---|---|---|
| Project phase | Section of time with a verifiable result and a milestone | HOAI work stage 4, approval planning, ends with the building application |
| Process group | Management processes that repeat in every phase | Planning, monitoring and controlling also run during construction |
| Project life cycle | Sequence of all phases of a project | Definition, planning, execution, closure |
The Project Management Institute has arranged these terms in its PMBOK Guide over the years:
- 6th edition (2017): It organizes project management into five process groups (initiating, planning, executing, monitoring and controlling, closing) and states that process groups are not project phases. The process groups repeat in every phase.
- 7th edition (2021): It replaces the process groups with 12 principles and 8 performance domains.
- Process Groups: A Practice Guide (2022): This companion volume continues the five process groups with 49 processes for predictive work (PMI, Process Groups Guide).
- 8th edition (November 2025): It brings the five groups back as "focus areas", alongside 6 principles and 7 performance domains (PM Study Circle, PMBOK 7 vs. 8).
The German standard DIN 69901-2 also describes five project management phases: initiation, definition, planning, control and closure. It means groups of management processes, not fixed sections of time.
What Are the Phases of a Project?
A project has as many phases as its life cycle defines. The most common models are the 4 phase model with definition, planning, execution and closure, and the 5 phase model, which treats monitoring as a separate phase. Industries such as construction use their own models, for example the 9 HOAI work stages in Germany.
4 Phase Model
The 4 phase model splits a project into these phases:
- Project definition
- Project planning
- Project execution and controlling
- Project closure
The names vary from source to source; the principle stays the same.
Project definition: In the first phase you set the project goals and work out opportunities and risks. Market and feasibility studies show whether the project can succeed. From the goals you derive the work packages. A business case assesses cost, scope and time frame. Only then do you decide whether the project moves into planning.
Project planning: In the second phase you create the project schedule or milestone plan. You schedule the tasks of each phase on a timeline; milestones mark the transitions between phases. Team members get an overview of the project and their tasks. You define communication channels, quality and risk management, and budgets.
Project execution and controlling: The team completes the tasks and work packages. Controlling compares plan and actuals continuously so that you spot deviations early and take countermeasures. Communication in the team decides whether deadline and quality hold.
Project closure: In the final phase the team completes open tasks, acknowledges the results and records the lessons learned. A closing meeting collects the experience; a short summary makes it usable for future projects.
5 Phase Model
The 5 phase model uses the names of the five PMI process groups as phases:
- Initiating
- Planning
- Executing
- Monitoring and controlling
- Closing
Read as a phase model, it separates monitoring from execution. Strictly speaking this is a simplification: in the PMBOK Guide, monitoring and controlling runs alongside all other groups, not after them.
What Is the Difference Between the 4 and the 5 Phase Model?
The difference lies in controlling: the 4 phase model integrates it into execution, the 5 phase model treats monitoring and controlling as a separate section. In practice you monitor throughout execution in both models.
| Phase (4 phase model) | Goal | Typical result | Milestone at the end | Equivalent in the 5 phase model |
|---|---|---|---|---|
| Definition | Decide whether the project is worth it | Project charter, goals, feasibility | Project charter approved | Initiating |
| Planning | Define how to reach the goal | Work breakdown structure, schedule, budget | Plan approved | Planning |
| Execution and controlling | Produce the result and manage deviations | Deliverables, status reports | Acceptance granted | Executing plus monitoring and controlling |
| Closure | Close the project properly | Final report, lessons learned | Project closed | Closing |
Example: What Do Project Phases Look Like in a Construction Project?
In German construction, the 9 work stages (Leistungsphasen) of the HOAI define the phases; Section 34 (3) HOAI weights them for buildings with fixed fee shares from 2% to 32%. If you map the work stages to the four project phases, you see where the effort lies.
2%
7%
15%
3%
25%
10%
4%
32%
2%
| Project phase | HOAI work stages | Fee share | Milestone at the end of the phase |
|---|---|---|---|
| Definition | 1 Basic evaluation, 2 Preliminary design | 2% + 7% = 9% | Preliminary design approved by the client |
| Planning | 3 Design, 4 Approval planning, 5 Detailed design, 6 Tender preparation, 7 Tender and award | 15% + 3% + 25% + 10% + 4% = 57% | Construction contracts awarded |
| Execution | 8 Site supervision | 32% | Building accepted |
| Closure | 9 Aftercare | 2% | Defect assessment completed |
Source of the percentages: Section 34 HOAI, gesetze-im-internet.de. The sum is 100%.
What the example shows:
- The planning phase carries the project: 57% of the base fee is earned before ground is broken. A phase plan that only shows construction time hides more than half of the work.
- Phases end with decisions: Before stage 8, the construction contracts must be awarded. This milestone is a gate: without it, execution does not start.
- Process groups run across phases: In stage 8 you still plan, monitor and control, for example the dates and costs of the trades. That is not a separate section of time.
The guide Project Management for Architects shows how architecture firms manage the HOAI stages day to day.
Why Is It Worth Defining Project Phases?
Project phases reduce complexity, create fixed checkpoints for quality and budget and give the team shared orientation. These three benefits apply to every project, regardless of size and industry.
1. Better project control
At the start of a project, the phase structure comes before the schedule in the Gantt chart. From the phases you derive the timeline more easily.
Projects consist of many tasks and work packages. Phases reduce this complexity: the project stays the same, but you focus on the activities of the current phase. Stakeholders, too, prepare more easily for a single phase than for the whole project.
2. Checking success
Every project phase ends with a milestone. On this fixed date you check whether the phase goal is reached and how overall progress looks.
Milestones are usually also the dates on which the client or steering committee reviews project status. That motivates the team to finish results by then.
For quality management, milestones are reference points: you assess the results so far and correct the next steps. You catch errors before the project ends, and the potential damage stays smaller.
3. Supporting communication
The team needs an overview of project status at all times. A visual representation of the phases shows everyone where their work stands in the project. In meetings it serves as shared orientation, and visible progress motivates.
How Do Project Phases Work in Agile and Hybrid Projects?
In hybrid projects, phases form the outer frame with milestones and approvals, and the team works iteratively inside the phases. A typical setup is a traditionally planned definition and closure phase and an execution phase in sprints or on a Kanban board.
Purely agile approaches such as Scrum do not use phases in the classic sense; they plan in short iterations with a result at the end of each sprint. As soon as budget approvals, contracts or permits set fixed dates, however, you need milestones again. The article on the waterfall model in project management shows the classic counterpart.
How Do You Represent Project Phases in Merlin Project?
In Merlin Project you represent each project phase as a first-level group in the Work Breakdown and place a milestone at the end of each group. Groups calculate their duration automatically from the activities they contain, so the phase duration is always up to date.
- Create phases as groups: In View > Work Breakdown, create one activity per phase, for example "Definition" or "Stage 1 Basic evaluation". Move the related activities below it with Indent (Ctrl+Cmd+→). The parent activity becomes a group.
- Place a milestone at the end of the phase: Use Insert > Milestone (Alt+Cmd+M) to add a milestone at the end of each group, for example "Construction contracts awarded". Milestones have no duration and appear as diamonds in the Gantt chart.
- Link the phases: Connect the milestone of one phase to the first activity of the next phase with Structure > Link > End to Start (Ctrl+Cmd+L). If one phase moves, Merlin Project moves the following ones too.
- Check the phase gates: The project view Milestones in the Work Breakdown shows milestones only, in other words your gates.
- Present the phases: In the Net Plan (Cmd+2), the project view Phases shows the first-level activities as phase arrows with title, expected start, expected end and completion.
Phases and milestones in the Work Breakdown
The sample project Office Move from the Merlin Project manual is split into three groups: Prepare old building, Prepare new building and Carry out move. After each group, a milestone marks the end of the phase: Old building cleared, New building ready and Move completed. Read more about the work breakdown structure in Merlin Project and traditional project management in the learning path.
For building projects, the templates for architecture and construction provide ready-made project structures. The Net Plan and the Phases view are part of Merlin Project, not Merlin Project Express.
Conclusion
Project phases split a project into sections with a verifiable result and a milestone. The 4 phase model is enough for most projects; the 5 phase model separates monitoring only on paper. Keep phases and process groups apart: phases follow one another, process groups repeat in every phase. The construction example shows how unevenly the effort is distributed: 57% of the HOAI fee falls into planning.
If you have any questions about this blog article or would like to discuss it, we look forward to your contribution in our forum.
Frequently asked questions
What are the phases of a project?
The common 4 phase model splits a project into definition, planning, execution with controlling and closure. The 5 phase model names initiating, planning, executing, monitoring and controlling, and closing. Industries have their own models, construction in Germany for example the 9 HOAI work stages.
What is a project phase?
A project phase is a time-bound section of a project with a defined, verifiable result. It ends with a milestone at which the client or steering committee reviews the result and releases the next phase.
Are the five PMBOK process groups project phases?
No. Initiating, planning, executing, monitoring and controlling, and closing are groups of management processes that repeat in every project phase. The PMBOK Guide 7 from 2021 replaced them with principles and performance domains; the Practice Guide from 2022 and the PMBOK Guide 8 from 2025 continue them.
What is a project phasing plan?
A phasing plan shows the phases of a project in sequence with their start and end dates and the milestone that closes each phase. In Merlin Project you build it from first-level groups and milestones and present it in the Phases view of the Net Plan.
How many work stages does the HOAI have?
The German HOAI has 9 work stages, from basic evaluation to aftercare. For buildings, Section 34 HOAI weights them with 2, 7, 15, 3, 25, 10, 4, 32 and 2 percent of the fee.
