One of the biggest challenges that project managers face during the planning phase is estimating the time and resources required accurately. It is very difficult to predict how long a task may take or what resources will be needed ahead of time without actually starting the work. Many factors like complexity of work, dependencies between tasks, availability of resources can impact timelines but may not be evident during planning. Overly optimistic time and resource estimates are a major reason for project delays. It requires experience from previous similar projects and factoring in contingencies to come up with realistic estimates.

Getting team members to participate actively in the planning process can also be difficult for project managers. People are often busy with their day to day work and may not see the need to spend time on planning. It takes persuasion from the project manager to get stakeholders, subject matter experts and future project team members involved in identifying requirements, evaluating options, assessing risks etc. Without their inputs, the plan may lack critical information or buy-in from the resources. Status updates are also needed to track progress against the plan which adds workload. Getting overwhelmed team members to prioritize planning activities is a challenge.

Budget constraints are a common issue faced during project planning. Stakeholders often have expectations of delivering more with less. It requires balancing features with what is feasible within the approved budget. Unexpected costs also come up during detailed planning. Trade-off discussions need to happen to agree on reducing or removing scope, adding funds or finding cost savings to stick to the allocated budget. Obtaining budget approvals for additional unexpected costs can delay the launch of some projects.

Agreeing on realistic deadlines with stakeholders is another area of challenge for project managers. Business objectives and external factors drive deadline expectations which may not match what detailed planning reveals. There is pressure to compress schedules to unrealistic timeframes despite quality or risk implications. Negotiation skills are needed to manage stakeholder expectations of when the project can realistically be delivered. Changes in priority during the planning stage can also disrupt timelines that were already tentatively agreed upon.

Lack of information and unclear requirements pose a major risk during initial planning. Not all details are known upfront. Scope may not be well defined or may change from the initial understanding. Subject matter experts may provide incomplete or inconsistent information. Dependencies with external factors or other projects may not be properly documented. This leads to gaps or ambiguity in requirements that become apparent only as planning progresses. Re-work is needed to revise plans as new information emerges or requirements stabilize which impacts timelines.

Integration with other related projects also poses coordination challenges. Projects may haveoverlapping tasks, resources or timeline dependencies that need to align during planning. Communicating and resolving interface issues takestime and effort. Getting visibility and buy-in across multiple project managers adds complexity. Late changes in related projects can disrupt plans that were already synchronized.

Establishing clear roles and responsibilities within large complex projects is another hurdle during planning. Different functional units, vendors, virtual teams may be involved. Individual competencies need mapping to specific work packages. Lines of communication and decision making need defining upfront to avoid confusion later. Internal politicking can delay finalizing accountabilities if not managed carefully by the project manager. Last minute additions of new team members without clarity on handover also poses disruptions.

In summary,project managers face significant challenges like inaccurate estimating, lack of team participation, budget constraints, unrealistic deadlines, unclear requirements, coordination across projects, defining roles that need to be carefully managed during the crucial planning stage to set the project for success. Experience, stakeholder engagement, contingency planning and change management are keys for project managers to overcome these challenges.