Primary vs excess layer professional indemnity: how layered PI towers work
Reviewed by Apex Insurance Brokers · Last reviewed 2026-08-05
Why firms build a PI tower at all
Professional indemnity (PI) insurance protects your firm against claims that you were negligent in the professional services or advice you provided. For many small firms, a single policy with a £1m or £2m limit is enough. But some businesses face potential losses far larger than any one insurer wants to write on its own.
That is where layering comes in. Instead of asking one insurer to carry, say, £10m of exposure, you buy a smaller primary policy and then stack excess layers on top until you reach the total limit you need. Each insurer takes a defined slice of the risk. The result is a single tower of cover that behaves, from a claim's point of view, much like one large policy.
Primary layer: the policy that pays first
The primary layer is the foundation. It responds from the ground up — the first pound of any covered claim comes out of this policy (subject to your excess or deductible). The primary insurer also usually leads on claims handling, defence costs and the day-to-day relationship, so the wording here matters most.
Key things the primary policy sets for the whole tower:
- The insuring clause and definitions — what counts as a claim, what "professional services" means for your firm.
- The self-insured excess — the amount you pay on each claim before cover starts.
- Whether limits are "costs in addition" or "costs inclusive" — that is, whether defence costs eat into your limit or sit on top of it.
- Aggregation and reinstatement — whether the limit is any-one-claim or in the aggregate for the policy year.
Excess layers: cover that only kicks in higher up
An excess layer (sometimes called an excess-of-loss layer) provides no cover at all until the layer beneath it is fully used up. If your primary limit is £2m, a first excess layer of £3m "excess of £2m" starts paying only after that first £2m has been exhausted, and then pays up to a further £3m.
Excess policies are typically cheaper per pound of cover than the primary, because the chance of a claim reaching that high is lower. Most excess wordings are written to "follow form" — they adopt the terms of the primary policy so the whole tower responds consistently. Where an excess layer differs from the primary, gaps can open up, which is exactly what a broker checks for.
How the layers stack: a worked illustration
The figures below are illustrative options, not a quote. They show how a £10m tower might be assembled for a firm that needs a high limit.
| Layer | Cover provided | Responds when |
|---|---|---|
| Primary | £2m | From the first pound (after your excess) |
| 1st excess | £3m excess of £2m | Once the first £2m is used up |
| 2nd excess | £5m excess of £5m | Once the first £5m is used up |
| Total tower | £10m | Combined limit available for one large claim |
A claim of £6m would draw £2m from the primary, £3m from the first excess and £1m from the second, leaving £4m of the tower still available for other claims that year.
Need a high limit that contracts or regulators demand? We build and place the whole tower for you.
Get a PI quote →Who typically needs a layered tower
Layered PI is common where individual contracts, professional bodies or clients require limits well above what a single insurer will comfortably provide. That often includes:
- Construction consultants, engineers and architects working on large or high-value projects.
- Solicitors and accountants who must meet minimum-limit rules set by their regulator or professional body.
- IT, technology and consultancy firms whose client contracts specify high indemnity limits.
- Larger firms whose fee income and project values create exposure a £5m single policy would not cover.
If a contract you are bidding for stipulates a limit you cannot meet with your current policy, a tower is usually how you close the gap. Speak to a broker before signing — you can start a quote here and we will map the layers to the requirement.
Where towers can go wrong
Because a tower is several policies acting as one, the joins are where problems hide:
- Non-concurrent wordings. If an excess layer does not truly follow the primary, an exclusion in the excess can leave a gap mid-tower.
- Different renewal dates. Layers renewing at different times can drift out of alignment. Most brokers keep the whole tower on one date.
- Aggregate erosion. Several claims in a year can quietly use up lower layers, reducing what is left for a later loss.
- Costs treatment. If the primary is "costs inclusive" but an excess is "costs in addition", the real cover available is not what the headline limits suggest.
This is the practical value of a broker: making sure every layer is concurrent, correctly ordered and priced so the tower behaves as one policy when a claim actually lands.
Common questions
Does an excess layer pay defence costs before the primary is exhausted?
Generally no. An excess layer only responds once the limit beneath it is used up, and that usually includes defence costs where the wording is "costs inclusive". Until then, the primary insurer handles and funds the defence. Always check how each layer treats costs.
Is buying layers cheaper than one large single policy?
Often, yes. Higher excess layers price at less per pound of cover because the likelihood of a claim reaching that height is lower. Layering also spreads the risk across insurers, which can make very high limits available that no single insurer would write alone.
Can different insurers sit on the same tower?
Yes, and they usually do. Each layer can be placed with a different insurer. What matters is that the wordings are concurrent and the layers are correctly stacked, so the tower responds as a single, seamless limit when you claim.
Apex Insurance Brokers Limited is authorised and regulated by the Financial Conduct Authority (FRN 724952). This guide is general information, not advice on a specific policy or a substitute for your policy wording.