1. Start with the real decision
Each tool begins with a narrow user question: whether a grant appears applicable, how two running costs compare, what capacity is sensible, or which official steps apply. Inputs are included only when they can materially alter the answer or an important caveat.
2. Prefer official and primary sources
Government agencies, regulators, scheme owners and provider price schedules are the first choice. Sources are linked on the tool page so the visitor can check the underlying guidance. See the source and review policy.
3. Separate facts, assumptions and estimates
Scheme values and eligibility rules are treated differently from planning assumptions such as annual solar yield or typical appliance use. A result should name the important assumptions and avoid implying that an estimate is an official determination or a quote.
4. Return the useful result after interaction
Tool explanations are rendered as normal indexable pages. Detailed rules, tables and matching logic are kept out of public browser bundles. Validated inputs are sent to the server, which returns only the personalised result needed for that interaction.
5. Test boundaries, not only happy paths
Calculation tests cover representative answers and critical limits. Forms enforce sensible ranges on both the browser and server. Results are also checked for small-screen readability, keyboard operation and clear error states.
6. Keep commercial influence outside the answer
A commission must never change a formula, eligibility result or ranking. Where a relevant commercial option is introduced in future, it belongs after the impartial result and must be clearly labelled.