Product design is the work of deciding what to build, for whom, and in what order, expressed through the interface. That makes it closer to product management than to visual design, which is why hiring a visual designer to do product design produces something that looks considered and still solves the wrong problem.
Software product design: the job is deciding, not decorating
A product designer's most valuable output is often a smaller feature set: understanding that two proposed features are one, that a third solves a problem nobody has, or that the real obstacle is in a flow nobody was looking at. That work saves engineering months and rarely appears in a portfolio. When you evaluate candidates or agencies, ask what they talked a previous client out of building, and what happened as a result.
Digital product design and development together, or handed over
Combined design and development engagements avoid the gap where a design is specified without regard to what it costs to build and is then silently changed by developers. They also concentrate dependency in one supplier. The workable middle is separate suppliers with an enforced conversation: design reviewed by the engineers who will build it, and a component based deliverable rather than unique screens, so implementation is assembly rather than interpretation.
Prototype the risky part, not the pretty part
Prototypes are worth their cost when they test something you are genuinely uncertain about: whether people understand a concept, whether a flow is completable, whether a complex interaction is usable. They are close to worthless as a way of showing stakeholders something attractive. Ask what the prototype is testing and what result would change the plan. If there is no answer that would change anything, it is a demonstration rather than a test.
Design decisions carry obligations in regulated products
In regulated products, some parts of the interface are not freely designable: required disclosures have rules about content, timing and prominence, and audit and access requirements such as the HIPAA technical safeguards at 45 CFR 164.312 shape flows as well as back ends. A product designer working in these areas needs to know where the constraints are, because discovering them after the flow is designed means redrawing it.
Questions people ask about digital product design
How is a product design consultant different from a UI designer?
Product design decides what to build, for whom and in what order, expressed through the interface. UI design is the visual and interaction layer. Hiring the second to do the first produces something considered-looking that solves the wrong problem.
Should design and development come from one supplier?
It avoids the gap where designs are specified without regard to build cost, and concentrates dependency. The middle path is separate suppliers with design reviewed by the engineers who will build it and a component based deliverable.
When is a prototype worth paying for?
When it tests something you are genuinely uncertain about and a result would change the plan. If no outcome would change anything, it is a stakeholder demonstration rather than a test, and it should be priced as one.