Applying the climate change allowances tables correctly

Flood risk climate change allowances are not one number. They are a set of percentage uplifts published by the Environment Agency, selected by where your site drains, how long the development will last, and how vulnerable the proposed use is. Pick the wrong combination and the flood risk assessment gets returned, usually with a one-line comment that costs three weeks.

The method is the same whether you are building one house on a plot or a small scheme. Find the catchment. Decide the lifetime. Read the allowance for the right percentile. Run the test twice, once for river flow and once for rainfall. Write down which version of the tables you used.

Where the allowances live and why they move

The allowances sit in the Environment Agency guidance page on gov.uk titled flood risk assessments: climate change allowances. They are not in the National Planning Policy Framework and they are not in the Planning Practice Guidance, although the PPG flood risk chapter tells you when to apply them.

They change. The Agency has reissued the tables several times, and a reissue has in the past changed both the values and the way they are grouped, moving from broad regional figures to allowances keyed to smaller units. A flood risk assessment written against superseded tables is not wrong in method, but it is wrong in arithmetic, and a lead local flood authority or the Agency will say so.

So treat the tables as a dataset with a publication date rather than as a fixed fact you can carry between projects. The same discipline that applies to version control on the biodiversity metric applies here.

Finding your management catchment and river basin district

Two different geographies are in play and people mix them up.

Peak river flow allowances are published for management catchments, which are the units the Agency uses for river basin management planning. Peak rainfall intensity allowances are published at river basin district level. A site in the Thames river basin district might sit in one of many management catchments within it, so the two lookups give you two different answers from two different tables.

Get the catchment from the Agency's own mapping rather than from a guess based on the nearest river. A site can be a few hundred metres from one watercourse and drain to another. Where the site sits on a catchment boundary, or the watercourse that floods it is not the one the postcode suggests, say so in the assessment and state which catchment you have taken and why. An assessor will accept a reasoned choice. An unstated one reads as an error.

Peak river flow against peak rainfall intensity

These answer different questions and both are usually needed.

The peak river flow allowance applies to fluvial flood risk: how much higher the river's peak flow may be in future, which feeds into modelled levels and therefore into finished floor levels and flood zone extents. The peak rainfall intensity allowance applies to surface water and to drainage design: how much more intense a short-duration storm may be, which feeds into attenuation volumes and pipe sizing.

A small residential scheme in Flood Zone 1 with no fluvial influence may need only the rainfall uplift, applied through the drainage calculations. A site in Flood Zone 2 or 3 needs both, because the river levels set the zone and the rainfall uplift sets the surface water design. If your Flood Risk Assessment applies one and stays silent on the other, expect a question.

Rainfall uplifts also drive the storage volumes in a Drainage Strategy, and the two documents have to use the same figure. Disagreement between a flood risk assessment and a drainage calculation on the same site is one of the quickest ways to lose credibility with a consultee.

Choosing an epoch from the development's lifetime

The tables give allowances for future time periods rather than for a single future year. You choose the period by working out how long the development is expected to last, starting from when it is built.

The PPG flood risk chapter sets the convention. Residential development is assessed over a lifetime of 100 years unless there is good reason to use a different figure. Non-residential development is usually assessed over a shorter lifetime, commonly 75 years, again subject to justification. A house given permission now therefore runs well into the next century, which puts it in the later of the published periods rather than the nearer one.

Two mistakes follow from this. The first is using the nearer period because the numbers are smaller. The second is using the development's expected commercial life, or the length of a lease, as though that were the planning lifetime. Neither survives contact with a reviewer.

The percentile is a separate choice. The tables publish more than one allowance for each catchment and period, and the flood risk chapter of the PPG directs which to test according to the flood zone and the flood risk vulnerability classification of the proposed use. More vulnerable and highly vulnerable uses in higher zones are tested against the more onerous allowances. Read that table rather than defaulting to the middle column.

Sea level and coastal sites

Coastal and tidal sites use a different part of the guidance. Sea level allowances are published as a rate of rise for regions of the English coast, cumulative over the period you are assessing, and they combine with allowances for offshore wind speed and extreme wave height where those matter to the site.

For a tidal site the calculation is additive over the lifetime rather than a single percentage uplift, which changes the shape of the working. Set it out as a table in the assessment with the start year, the end year, the annual rate and the total. A reviewer can check that in thirty seconds, and a reviewer who can check your arithmetic quickly tends to accept it.

Sites influenced by both river and tide need the joint probability approach, which is specialist modelling work rather than a lookup. That is the point at which a competent flood risk modeller earns their fee.

Recording the version you used

Write a short methodology paragraph that states, in order: the guidance page title, the date of the version you consulted, the management catchment, the river basin district, the development lifetime and the year it runs to, the percentile applied, and the resulting percentage for each of river flow and rainfall.

That paragraph is six lines and it prevents most of the correspondence that follows a submission. It also protects you if the tables are reissued between submission and determination, because it shows the assessment was current when it was written.

Keep a dated copy of the tables in the project file. Government pages are updated in place, so the figure you read in March may not be the figure on the screen in September, and the page history will not always make the change obvious.

Errors that come back from the consultee

The recurring ones, in rough order of frequency:

• Applying a river flow allowance to a rainfall calculation, or the reverse.
• Using a superseded set of tables without saying which set was used.
• Assessing a residential scheme over a 60-year or 75-year lifetime.
• Taking the central allowance for a use that the vulnerability table says should be tested against a higher one.
• Applying the uplift to the modelled level instead of to the flow, then presenting the result as a modelled output.
• Quoting the allowance in the flood risk assessment and a different one in the drainage calculations.
• Ignoring the allowance altogether on a Flood Zone 1 site of one hectare or more, where surface water still has to be assessed.

None of these are difficult to avoid. They happen because the allowance is picked once, early, by someone working from a previous project, and never revisited when the scheme or the guidance moves.

Before you start, order the drainage evidence you will need alongside the allowances. Sewer records from a pre-development enquiry take time to arrive and they determine whether the discharge strategy you are modelling is available at all.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.