What Is an Assumptions Map and How It Prevents Costly Mistakes

Teams rarely fail because they did not work hard. They fail because they invested time and budget based on things that felt true, but were never checked.

An assumptions map is a simple tool that helps you surface what you are treating as true, agree what matters most, and test it early. It is especially useful when you are starting a new project, designing a service, writing a bid, or making a change that affects customers, staff, or partners.

What an assumptions map is

An assumptions map is a visual way to capture the beliefs behind a plan. It helps you separate facts from guesses, and it makes hidden risks visible.

Assumptions usually sit under statements like:

1. We think people will use this.

2. We think this will save time.

3. We think partners will share data.

4. We think procurement will allow it.

Why assumptions cause costly mistakes

When assumptions are not made explicit, teams often build the wrong thing, solve the wrong problem, or design something that cannot be delivered in the real world.

The cost is not just financial. It shows up as rework, delays, loss of confidence, and strained relationships with stakeholders.

When to use an assumptions map

Use it any time you are about to commit resources and there is uncertainty. Common moments include:

• Before writing a business case

• At the start of a project or programme

• When shaping a new service or offer

• Before responding to a tender or partnership opportunity

• When a project is stuck and there are competing opinions

The simple assumptions map structure

You can do this on a whiteboard, in a shared document, or on a single slide.

Step 1. Write the plan or idea in one sentence.

Step 2. List the assumptions that must be true for it to work. Aim for 10 to 20.

Step 3. Sort each assumption into one of four types.

• User or customer assumptions

• Operational assumptions

• Commercial or funding assumptions

• Stakeholder or policy assumptions

Step 4. Rate each assumption on two scales.

• Importance. If this is wrong, does the plan fail or just need tweaking?

• Uncertainty. Do we know this is true, or are we guessing?

  
  

Step 5. Prioritise the assumptions that are both high importance and high uncertainty. These are your riskiest assumptions.

How to test assumptions quickly

Testing does not need to be expensive. The goal is to learn early, before you scale.

Here are simple tests that work in most organisations.

• Short interviews with the people affected

• A small pilot with one team or one location

• A process walk through to see what actually happens day to day

• A data check using existing reports or service metrics

• A pre mortem session to pressure test delivery risks

Copy and paste template

[Use this in a project brief or workshop.

Plan in one sentence:

Assumptions list:

Most important and uncertain assumptions:

How we will test them this month:

What would change if we are wrong:]

Example assumptions map

Plan in one sentence: Introduce a new referral pathway so people can access support faster.

Assumptions list: People will understand the new pathway, frontline staff will use it consistently, partners will accept referrals, data sharing agreements will be in place, demand will not exceed capacity, and the pathway will reduce repeat contacts.

Most important and uncertain assumptions: Partners will accept referrals within agreed timescales, staff will adopt the new process without additional training time, and the pathway will reduce repeat contacts.

How we will test them this month: Run a two week pilot with one team, speak to 10 staff members and 10 service users, and review baseline data on repeat contacts.

What would change if we are wrong: Adjust the pathway steps, agree a simpler handover with partners, and update capacity planning before rolling out.

Common mistakes to avoid

The biggest mistake is treating assumptions as opinions to debate rather than risks to test. Another is listing assumptions but not prioritising them, which leads to a long list and no action.

It is also easy to test the comfortable assumptions and avoid the hard ones. If an assumption feels politically sensitive, it is usually the one you most need to check.