Two different questions
A diagnosis answers one question: what condition does a person have? Functional capacity answers a different one: how does that condition affect what they can actually do, day to day? Both matter — but they aren't the same thing, and for the NDIS they aren't interchangeable.
Diagnosis tells you what a person lives with. Functional capacity tells you how it affects their everyday life — their mornings, their cooking, their work, their relationships, and how often they need support.
The NDIS funds support, not labels
The NDIS is designed around functional capacity because that's what determines support needs. Its decisions rest on whether a disability substantially affects daily functioning (Section 24 of the NDIS Act) and what supports are reasonable and necessary as a result (Section 34). A diagnosis can establish that a condition exists, but on its own it can't show how much support a person needs — because that depends on function, which varies enormously from one person to the next.
Two people, the same diagnosis, very different days
This is the heart of it. Two people can share the exact same diagnosis and have completely different functional capacity — different strengths, different barriers, different support needs.
Person A
Manages self-care and holds down work independently, but finds social situations and unexpected change overwhelming, and needs support there.
Person B — same diagnosis
Needs daily support with routine, meals and leaving the house, and relies on a carer for much of the day.
A diagnosis alone can't tell these two people apart, or show a planner what each one actually needs. Functional capacity can — and that's the gap an assessment is there to fill.
Why a diagnosis letter isn't enough
For an NDIS application or review, a diagnosis is often necessary but rarely sufficient. A letter confirming a condition tells a planner what you have — not how it affects your day, what you can manage alone, what you can't, or how often you need help. Without that functional picture, there's little for a planner to base support decisions on.
A diagnosis says what. The NDIS needs to know how, and how much — and that's a question about function.— Nisha Bal, Seeds OT
An FCA turns a diagnosis into a functional picture
This is exactly what a Functional Capacity Assessment provides. It takes your diagnosis as the starting point and builds out the functional detail around it — grounded in validated tools and clinical reasoning — across the major domains of daily life. For each one, it captures not just whether you're affected, but how, and how often.
- Self-care and personal routines
- Daily tasks — cooking, household, getting around
- Communication and social participation
- Work, study and community life
- Managing emotions, behaviour and change
What this means for your evidence
If you're preparing for the NDIS, the practical takeaway is simple: gather what shows function, not just what names a condition. Diagnostic letters are worth including, but the strongest evidence shows how your disability affects daily life. An FCA is built to capture exactly that — which is why it can carry so much weight in an application or plan review.
Frequently asked
Usually not on its own. A diagnosis can help establish that a condition exists, but the NDIS bases support decisions on functional impact — how the condition affects your daily life. That's why a functional assessment is so often needed alongside diagnostic information.
Yes — because support follows function, not the label. Two people with the same diagnosis can have very different strengths, barriers and support needs, and the NDIS funds supports according to each person's actual functional capacity.
No. An FCA is not a diagnostic assessment. It works from your existing diagnoses and focuses on function — describing how your condition affects daily life and what support follows, rather than naming or changing a diagnosis.
Include diagnostic letters, but prioritise anything that shows function — therapy notes, school reports, support worker observations, previous assessments. An FCA then pulls this together with structured interviews and validated tools into a clear functional picture.