Design Requirements and Specifications
What Are Design Requirements?
Before any design work begins, an engineer must establish what the product actually has to achieve. Design requirements are the full set of conditions the product must satisfy. They come directly from analysing the brief, researching the user, and understanding the context of use. Getting the requirements right is critical: a product that is beautifully made but fails to meet its requirements is a failed product, regardless of its technical quality.
Needs and Wants
Design requirements are divided into two categories. Needs are non-negotiable: they are the essential criteria without which the product simply cannot fulfil its purpose. A bicycle helmet needs to protect the user's head in a collision — that is a need. Wants are desirable but optional features that would improve the product without being essential — for example, the same helmet being available in multiple colours. In a specification, needs take priority over wants, and wants are usually only pursued if time and budget allow.
The distinction between needs and wants matters in the exam because you may be asked to classify a list of requirements or to argue why a particular criterion is a need rather than a want. Always justify your classification by referring back to the product's core function and the safety or performance consequences of omitting that feature.
Needs are essential — without them the product cannot fulfil its purpose or may be unsafe. Wants are desirable — they improve the product but their absence does not make it non-functional. In a design specification, needs must be satisfied before wants are considered. This hierarchy directly affects decisions about cost, time and scope during the engineering design process.
Quantitative and Qualitative Criteria
Design specifications contain two types of criteria. Quantitative criteria are expressed as numbers or ranges that can be measured objectively — for example, "the product must weigh no more than 500 g" or "the handle must fit hands in the 5th to 95th percentile range." Quantitative criteria make it straightforward to test whether a prototype meets the specification: either the measurement passes or it does not.
Qualitative criteria describe characteristics that cannot easily be reduced to a number — appearance, feel, style, and user satisfaction. For example, "the product must have a professional appearance suitable for an office environment" is qualitative. Qualitative criteria are harder to test objectively and typically require user feedback or expert judgment. A strong specification contains both types: quantitative criteria ensure functional performance; qualitative criteria ensure the product meets user expectations that numbers alone cannot capture.
Consider a specification for a new children's stacking toy. Quantitative criteria might include: maximum dimension of any piece = 40 mm (choking hazard regulation), minimum compressive strength = 50 N, production cost per unit = under £1.80. Qualitative criteria might include: bright, primary colours that appeal to children aged 2–5; pieces that feel satisfying to click together; a design that parents perceive as durable and safe. Neither type alone is sufficient. A toy that passes all quantitative tests but looks unappealing will not sell; a toy that looks perfect but fails the strength test is unsafe. The combination of quantitative and qualitative criteria in the specification gives the design team clear, complete targets to design and test against.
The Design Specification and ACCESS FM
The design specification is the document that translates all the requirements — needs and wants, quantitative and qualitative — into a formal set of criteria that the design must meet. The ACCESS FM framework provides the standard structure for organising specification criteria: Aesthetics, Cost, Customer, Environment, Size, Safety, Function, Materials and Manufacturing.
Each heading prompts specific questions. Aesthetics: what must the product look, feel or sound like? Cost: what is the maximum unit production cost? Customer: who will use it and what are their needs? Environment: will it be used indoors or outdoors; are there temperature, moisture or corrosion constraints? Size: what are the maximum and minimum dimensions? Safety: what hazards must be mitigated; which regulations apply? Function: what must the product do and how well? Materials: what material properties are required (strength, flexibility, food-safe, etc.)? Manufacturing: how will it be made; in what quantities; are there process constraints?
ACCESS FM is used at two points in the iterative process: first, as a framework for analysing existing products during secondary research (covered in Lesson 2); and second, as the structure for writing the design specification. Knowing that the same nine headings serve both purposes — and being able to apply them in both contexts — is a frequently examined point.
Using the Specification Throughout the Design Process
A design specification is not written once and forgotten. In the iterative process, the specification is a live reference document: every design idea is checked against it, every model is assessed against it, and every prototype is evaluated against it. When a prototype fails to meet a criterion, the designer knows exactly what to change. When all criteria are met, the designer has evidence that the product is a successful solution to the brief. This is why writing a precise specification — with measurable criteria where possible — is so important: vague criteria produce vague evaluation and no clear basis for decision-making.
Weak specification criteria are a common source of lost marks. "The product must be strong" is too vague to test — replace it with "the product must withstand a compressive force of at least 150 N without permanent deformation." "The product must look nice" cannot be evaluated — replace it with "the product must have a finish that users in a target focus group rate as aesthetically appropriate for a professional office environment." In exam questions that ask you to write or improve specification criteria, always aim for precision: include a number or a testable condition wherever possible, and name the ACCESS FM heading the criterion belongs under.