Housing site investigations: what to understand early

1. Start with the decision and the available evidence

Define what the work must help decide: whether to promote or acquire land, progress a layout, prepare a cost plan, approach a delivery partner, or determine whether more investigation is justified.

• Confirm the site boundary, ownership position and any known access or rights issues.
• Collect planning history, previous uses, existing surveys, ground records and utility information.
• Review environmental, flood, drainage, ecology, heritage and infrastructure constraints with the relevant advisers.
• Record what is known, what is assumed, the source and date of each input, and what remains uncertain.

2. Coordinate ecology and seasonal requirements

Ecology work can identify protected species, habitats, invasive plants and survey timing constraints that may affect the developable area or programme. Ask an ecologist to set out the evidence required, any seasonal windows, likely mitigation and the consequences for the emerging layout and sequence.

Where Japanese knotweed or another invasive non-native plant is suspected, identify it early and seek competent specialist advice. Avoid moving soil or plant material until a suitable management approach is agreed. Government guidance explains how contaminated soil or plant material can cause ecological harm and may be controlled waste; the correct treatment and disposal route depends on the material and current rules. Read the GOV.UK guidance on preventing Japanese knotweed from spreading and the current guidance on harmful invasive and non-native plants.

3. Investigate ground, contamination and remediation questions

Previous land use, made ground, geology, groundwater and visible site conditions may point to further work. A suitably qualified environmental and geotechnical team can define desk studies, intrusive investigation, laboratory testing and any remediation options needed for the proposed development.

For the commercial review, connect the findings to likely abnormal costs, material movements, foundation assumptions, risk allowances, programme and phasing. Do not treat an early desktop view as a substitute for intrusive investigation or a specialist costed solution.

4. Test drainage, utilities and access together

Ask the relevant designers and providers to check whether drainage strategy, outfall, attenuation, utility capacity, connection points, highway access and off-site works can support the intended scale and phasing. An unresolved connection or access dependency can affect the layout and delivery sequence as well as cost.

Bring constraints onto a coordinated plan and identify who will confirm each item, what design or survey is needed, and when the answer is required.

5. Carry the evidence into viability and delivery

Investigation findings only protect a project when they change the assumptions used to make decisions. Update the cost plan, risk register, development appraisal, phasing and programme as evidence emerges. Record ranges and sensitivities where costs or solutions are not yet confirmed.

• Separate evidenced costs from provisional allowances.
• Identify dependencies that could alter developable area, unit numbers or timing.
• Agree owners and deadlines for outstanding surveys, approvals and provider input.
• Revisit land value and viability when a material assumption changes.

For government planning guidance on how viability considers development value, cost and site-specific abnormal and infrastructure costs, see Planning Practice Guidance: Viability.

A practical early-stage checklist