Technical approval for highway structures on an estate

If your estate layout includes a retaining wall holding up an adoptable road, a culvert under it, or a footbridge over a watercourse, that structure has to go through technical approval before the highway authority will take it on. Technical approval is a separate process from the section 38 adoption agreement, it runs to its own timetable, and it is the item most likely to sit on the critical path of a first occupation date.

The process comes from the highway engineering world rather than the planning world. It is set out in the Design Manual for Roads and Bridges, which most local highway authorities adopt for their own structures with local amendments, and it exists so that the authority inheriting a structure knows it was designed, checked and built to a standard it can maintain for a very long time.

Which structures need approval

The trigger is not the presence of a structure but whether the authority will become responsible for maintaining it. Anything within the adoptable highway boundary that is not a road, a footway or a standard drainage run is likely to be caught.

• Retaining walls supporting or retaining the highway, commonly from around 1.5 metres of retained height, though authorities set their own trigger.
• Bridges and footbridges of any span.
• Culverts carrying a watercourse under an adoptable road, usually above a stated diameter.
• Headwalls, wing walls and outfall structures on adoptable drainage.
• Sign and lighting gantries, high masts and cantilever structures.
• Subways, underpasses and any buried structure beneath the carriageway.
• Reinforced soil and crib wall systems, which authorities treat cautiously because of the proprietary design element.

Structures outside the adoptable boundary avoid the process entirely, which is the single most useful fact here. A retaining wall set behind the highway boundary, on land that remains in private or management company ownership, needs building regulations attention and a maintenance arrangement but not technical approval.

The approval in principle document

The first formal step is an Approval in Principle, usually shortened to AIP. It is a stand-alone document submitted to the authority's structures team before detailed design starts, and it sets out how the structure will be designed rather than what it will look like.

An AIP typically records the design standards and codes to be applied, any departures from standard sought, the design life, the loading to be applied, the ground conditions and geotechnical parameters, the drainage arrangement, the durability and protective systems, the proposed maintenance regime, and the category of check that the designer considers appropriate.

The departures section is the one to get right. A departure from standard requires the authority's agreement, and where the standard belongs to another body that body's agreement may be needed too. Identifying a departure at AIP stage is routine. Discovering one during detailed design, after the AIP has been certified, means going back.

Authorities usually publish their own AIP form. Use it rather than a generic version, because the form encodes the local amendments and the boxes are what the checker works through.

Category of check and who does it

The amount of independent checking scales with the consequence of failure. The Design Manual for Roads and Bridges sets out categories running from a simple check by another member of the design team through to a full independent check by an organisation separate from the designer, with an independent analysis carried out from first principles.

The designer proposes the category in the AIP and the authority agrees or raises it. Simple structures with well-understood behaviour sit at the low end. Anything with unusual geometry, a proprietary system, an unusual loading case or a long span moves up, and so does anything where failure would close a road or endanger the public.

Category matters to programme and to cost. A high category check needs an independent checking organisation appointed, briefed and paid, and its queries have to be closed out before the design is certified. On a small estate that can add two to three months that nobody allowed for.

Design life and what it commits you to

Highway structures are designed for a long service life, and 120 years is the usual figure for bridges and major structures under the relevant standards. That single number drives a series of decisions with cost consequences.

Durability specification tightens. Concrete cover, waterproofing systems, bearing and joint selection, and corrosion protection all get specified to a level that a private development structure would not need. Access for inspection becomes a design requirement rather than an afterthought, which means inspection walkways, safe access to bearings and clearances for a bridge inspection unit.

And maintenance liability gets priced. Authorities take a commuted sum for the future maintenance of structures they adopt, calculated over the design life. A structure with proprietary components, difficult access or a short-life waterproofing system attracts a higher sum than a simple reinforced concrete wall with a long maintenance interval. Designing for maintenance is the most reliable way to reduce that payment.

How it interacts with the adoption agreement

Technical approval and the section 38 agreement are separate but coupled. The authority will not normally complete the section 38 agreement, or will not release the structure element of the bond, until the AIP is certified and the design check is closed out.

The sequence that works on most schemes runs like this.

1. Identify adoptable structures at layout stage, before the planning application is submitted.
2. Submit the AIP as soon as the layout is fixed, well before detailed design.
3. Agree the category of check and appoint the checker.
4. Complete detailed design and the independent check, and obtain design certification.
5. Complete the section 38 agreement, with the structure included in the bond and the commuted sum.
6. Build under supervision, with the authority's inspections recorded.
7. Provide as-built records, a health and safety file and the maintenance manual before the structure is certified complete.

Step seven is regularly forgotten and regularly delays adoption. The records package for a structure is more demanding than for a road, and assembling it retrospectively from a contractor who has left site is painful.

Where the works also touch the existing public highway, the technical approval process runs alongside the separate agreement covering those works, and the two use different bonds. The interaction with section 278 agreements and how they follow a permission is worth mapping on one programme rather than two.

Programme effect

Assume the AIP itself takes several weeks to be reviewed and certified, assume the independent check adds longer where the category is high, and assume at least one round of comments on each. Authorities' structures teams are small and are shared across all their work.

The practical consequence is that a structure identified late becomes the constraint on first occupation. A retaining wall spotted at detailed design stage, on a scheme where the roads are already built, can hold up adoption for a year.

Avoiding adoptable structures altogether

On many estate layouts the least costly structure is the one you do not build. Regrading the site to remove the level change, moving the road alignment away from the slope, putting the level change into private gardens rather than into the highway boundary, or leaving a retaining wall in management company ownership with a maintenance fund are all worth testing before the layout is fixed.

That test belongs at the same stage as the Transport Assessment or transport statement, when the access strategy and the internal layout are still movable. Once the planning permission fixes the layout, the structure is fixed with it. Getting the level strategy right also depends on knowing the ground, so a Geotechnical Assessment carried out before the layout is fixed earns its fee twice over.

Structural design and checking are engineering work requiring a suitably qualified and experienced engineer, and PlanScope's drafts support the planning submission rather than the structural design.

This is general guidance, not legal advice. Check the position with your local highway authority or a qualified professional before relying on it.

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