BSA 2022 s.135 and engineering PII: what changed for structural and design engineers

~5 min read

Reviewed by Matthew Bartlett, Director · Last reviewed 2026-07-06

Section 135 of the Building Safety Act 2022 amended section 1 of the Defective Premises Act 1972 to extend the limitation period for claims relating to defective residential dwellings — retrospectively to 30 years and prospectively to 15 years. The change applies to any professional whose work fell within the DPA scope, including structural engineers, civil engineers, mechanical and electrical engineers, and other design engineers who worked on residential dwelling projects. The engineering PII market has been shaped materially by the change, in ways that are both parallel to and different from the architects' market response. This entry sets out what section 135 did for engineers specifically, how the market has responded, and where engineers with residential exposure sit in 2026.

What the change did

The DPA 1972 imposes a duty on those taking on work in connection with the provision of dwellings — a duty owed to the person for whom the dwelling is provided and to subsequent owners. Before the BSA, the limitation period for a claim under DPA section 1 was six years from the date of completion. Section 135 of the BSA extended that limitation window: 30 years retrospectively (for dwellings completed before 28 June 2022) and 15 years prospectively (for dwellings completed after that date). Both changes are in force and apply to design engineers on the same footing as architects and contractors, provided their work fell within the DPA scope.

Which engineers are exposed

The scope test is factual: did the engineer's work relate to the provision of a dwelling? A structural engineer who signed off on the structural design of a residential apartment block is within scope. A civil engineer who designed the below-ground drainage and infrastructure for the same block is within scope where the drainage forms part of the dwelling provision. An M&E engineer who designed the heating, ventilation and mechanical systems is within scope where those systems form part of the dwelling. An acoustic engineer, a fire engineer, a facade engineer — each within scope where the work forms part of the dwelling provision.

The scope does not attach to purely commercial engineering work. A structural engineer working on office buildings, industrial warehouses, or retail schemes without any residential element sits outside DPA section 1 and therefore outside the extended limitation window. The most heavily exposed engineers are those with mixed portfolios containing significant residential dwelling work — and the majority of UK civil and structural consultancies fall into this category.

The retrospective element

The 30-year retrospective limitation is the harder change for engineers to absorb. An engineer who signed off on a structural design in 2000 for a residential block completed in 2001 now sits within a claim window running to 2031. Firms with 20 or 30 years of residential engineering work have found themselves reviewing files they had assumed were closed and having conversations with insurers about the shape of the exposure.

The market response

The engineering PII market repriced through 2022 and 2023, in parallel with the architects' market. Firms with heavy residential exposure saw premium increases in the 30-60% range, sometimes higher for high-rise residential specialists. Some insurers narrowed their appetite for engineering firms with residential specialism; the market did not see wholesale exits, but the pool tightened. By 2024-2025 the market had stabilised, with residential-exposed firms pricing at a persistent premium to non-residential firms.

What insurers now ask

Engineering PII proposals now ask, in effect, five things about residential exposure. First, the proportion of fee income over the last 30 years attributable to residential dwelling work. Second, the largest single residential project by construction value. Third, whether the firm has worked on high-rise residential (particularly above 18m — high-risk buildings under the BSA's new regulatory regime). Fourth, whether historical files are accessible and reviewable. Fifth, whether the firm has reviewed its historical residential work in the light of section 135 exposure.

The high-rise residential subset

Buildings above 18m (or above 30m in older regimes) attract the "high-risk building" definition under BSA 2022 and the new regulatory scrutiny administered by the Building Safety Regulator (part of the Health and Safety Executive). Structural engineers who signed off on high-rise residential designs during the retrospective window face an intersection of two exposures: the extended DPA limitation and the specific scrutiny applied to high-rise residential post-Grenfell. Insurers writing PII for firms with meaningful high-rise residential involvement price this subset materially above the general residential PII rate.

Run-off implications

The BSA change shifts the run-off decision for engineering firms materially. The market baseline for engineering run-off has historically been six years, matching the pre-BSA DPA limitation. For firms with material residential exposure — most structural and civil consultancies — six years is now demonstrably inadequate. Extended run-off of 12, 15, or 30 years is a serious conversation for engineering firms winding down with residential back-books. The cost delta is meaningful; the exposure it covers is more so.

Practical points for continuing engineering firms

Three practical points recur for firms continuing to trade.

First, historical file accessibility. Files from 2005 in physical archive need to be reachable if a claim surfaces in 2028. Digitisation of pre-2015 residential engineering records is a worthwhile investment for any firm with material historical residential involvement.

Second, current wording review. Firms whose PII wording was placed pre-BSA may not carry adequate provision for the extended-tail exposure. A wording review at the next renewal is a sensible step, particularly on the aggregation position and any defence-costs provisions.

Third, considered residential appetite going forward. Some engineering firms have deliberately narrowed their residential intake since 2022, on the view that the 15-year prospective claim tail changes the risk-adjusted return calculation. That is a defensible commercial position, particularly for firms with heavy historical exposure who wish to bound future accumulation.

Worked example

Illustrative only. A six-engineer structural consultancy with a mixed portfolio: 40% commercial multi-storey, 35% residential (including some pre-2015 medium-rise work and one 2018 high-rise-adjacent scheme at 22 metres), 25% civil-structural infrastructure. Fee income £1.9 million. Contractual matrix requires £10 million cover on the largest current appointments. Historical file review carried out in 2023 identifying approximately 45 residential schemes since 2010 with meaningful potential BSA-relevant exposure. Broker recommendation: £10 million primary layer plus £10 million top-up, explicit high-risk residential endorsement, 15-year run-off costed as a planning number for the partnership. Insurer pricing reflects the residential exposure but is broadly stable year-over-year given the well-documented review.