Design Stage vs As Built Reports: What You Need to Know

Two Reports, One Compliance Journey

Part G water efficiency compliance requires two reports for most new dwellings:

1. Design Stage report — submitted before construction, based on specified fittings
2. As Built report — submitted after construction, based on actually installed fittings

Both must demonstrate that the dwelling meets the target (125 or 110 L/p/d). Building Control will not sign off the project without both.

The Design Stage Report

Purpose

The Design Stage report demonstrates that the planned dwelling will meet the water efficiency target. It is based on:

• The architect's specifications
• Manufacturer data sheets for fittings and appliances
• The water reuse system design (if applicable)

When It Is Produced

The Design Stage report is typically produced:

• After planning permission is granted
• Before construction begins
• As part of the Building Regulations application

What It Includes

• Project details (address, client, assessor)
• Target limit (125 or 110 L/p/d)
• Plot-by-plot calculation of consumption
• Compliance status for each plot
• Assessor declaration

Key Considerations

• The report is only as good as the specifications — if fittings are changed during construction, the report may no longer be valid
• It is prudent to specify fittings with some margin below the target, so minor substitutions do not cause non-compliance
• The Design Stage report is a prediction, not a guarantee

The As Built Report

Purpose

The As Built report confirms that the completed dwelling meets the water efficiency target. It is based on:

• The actually installed fittings (verified on site)
• Any changes from the original design
• Commissioning data for water reuse systems (if applicable)

When It Is Produced

The As Built report is typically produced:

• After construction is complete
• After all fittings are installed
• Before final Building Control inspection

What It Includes

• The same structure as the Design Stage report
• Updated calculations reflecting installed fittings
• Compliance status for each plot
• Assessor declaration
• Reference to the original Design Stage report (if applicable)

Key Considerations

• The As Built report is the definitive compliance document
• If installed fittings differ from the design, the calculation must be updated
• If the As Built calculation exceeds the target, remedial action is required (fitting upgrades or water reuse installation)

Managing the Transition

Keep Specifications Consistent

The most common cause of As Built non-compliance is fitting substitution during construction. To avoid this:

• Clearly specify fittings in the design
• Communicate the water efficiency requirement to the contractor
• Flag any proposed substitutions for assessment before installation
• Maintain a record of installed fittings

Allow a Margin

If your Design Stage calculation is close to the target (e.g., 123 L/p/d against a 125 target), any minor change could push it over. Aim for a margin of at least 5–10 L/p/d below the target in the design.

Water Reuse Systems

If your design includes rainwater harvesting or greywater recycling:

• Ensure the system is installed as specified
• Commissioning data may be required for the As Built report
• The system must be operational for the savings to be credited

What If the As Built Fails?

If the As Built calculation exceeds the target, options include:

• Upgrading fittings — replacing high-flow taps or showers with more efficient models
• Installing water reuse — adding rainwater harvesting or greywater recycling
• Negotiating with Building Control — in rare cases, alternative compliance routes may be possible

Remedial work after construction is expensive — getting the design right and managing substitutions is far cheaper.

Generating Both Reports

Our Part G Water Calculator supports both Design Stage and As Built reports. For each project, you can:

• Set the report stage (Design or As Built)
• Enter the relevant fittings and systems
• Generate a PDF report for Building Control

When the project moves from design to construction, simply update the report stage and adjust the fittings to reflect what was installed.

Start a project and generate your compliance reports today.