Building near the coast: flood zones, foundations and salt air

By James Reilly, Swanbourne Construction Ltd · Published 15 October 2026 · Environment Agency advice and Arun flood data checked October 2026

Building an extension on the West Sussex coast is mostly about three things: flood risk, the ground, and the salt air. If your house is in Environment Agency Flood Zone 2 or 3 — as parts of Pagham, Bognor Regis, Middleton-on-Sea, Littlehampton and Selsey are — a planning application or prior approval needs a short flood risk assessment, and the Environment Agency now recommends new floors at least 600mm above the estimated flood level, or extra flood-resilience measures where that is not possible. Below ground, the coastal plain is a mix of silt, gravel and shrinkable clay with high winter groundwater, so foundations are designed after a trial hole, not before. Above ground, stainless fixings and coastal-rated finishes are what stop the job being redone in five years.

Almost all of our work is on this stretch of coast. None of what follows is a reason not to build here. It is a reason to price the footings after looking at the ground, and to think about water at the drawing stage rather than the first wet winter.

Flood zones: check your address first

The Environment Agency's Flood Map for Planning places every address in one of three zones. Zone 1 is low probability of flooding from rivers or the sea; Zone 2 medium; Zone 3 high. It takes a minute to check and it is the first thing to do.

Two things about the map surprise people:

• The zones ignore flood defences. A house behind a sea wall can still be in Flood Zone 3, because the map shows what would happen without the defence.
• Flood Zone 1 does not mean no flood risk. The zones are about rivers and the sea. Surface water and groundwater are separate risks, and on the coastal plain they are real ones — in January 2023 heavy rain on a high water table closed Pagham Road and Lower Bognor Road.

Arun District Council published a new Strategic Flood Risk Assessment, with a final report dated March 2026, replacing the 2016 version, with groundwater mapping and updated climate change data. It covers Arun outside the National Park. Chichester District Council has its own. Responsibility is split: the Environment Agency deals with the sea and main rivers; West Sussex County Council with surface water, groundwater and the rifes; Arun with planning and coastal erosion.

Do you need a flood risk assessment for an extension?

If your house is in Flood Zone 2 or 3 and the extension needs a planning application, the council will expect a flood risk assessment with it. (The larger home extension prior approval route is judged only on the neighbours' amenity, so it does not need one — but the floor-level advice below is still worth following.) For a householder extension it is short and proportionate, following the Environment Agency's standing advice for minor extensions. It covers:

• the site and what is proposed;
• the estimated flood level, including an allowance for climate change;
• the finished floor levels of the extension, measured against Ordnance Datum;
• the flood resistance and resilience measures to be used, shown on the drawings.

Floor levels: the advice changed in May 2026

The Environment Agency updated its standing advice on 28 May 2026. It now recommends that floor levels are at least 600mm above the estimated flood level, with flood-resistant construction up to at least that height — reducible to 300mm where the flood level is known with high certainty. Where floors cannot be raised, extra flood resistance and resilience measures are needed instead. Older guides, and plenty of websites, still say an extension's floor can simply be no lower than the existing house. That option is no longer in the current advice, so check the live page when the drawings are being done.

Raising an extension floor above the house floor has knock-on effects: a step at the knock-through, door thresholds, and roof heights against the permitted development limits. Resilience measures are the usual alternative where a step is unacceptable — water-resistant floor and wall finishes, raised sockets, non-return valves on drains, flood doors or barriers.

Other permissions near water

Work within 20m of a main river, a flood defence or a flow-control structure may need a flood risk activity permit from the Environment Agency, and work affecting a rife or ditch may need consent from West Sussex County Council. Planning permission does not cover either.

Pagham, the beach estates and coastal change

Pagham is where the coast decides most. The low land around the harbour lies largely in Flood Zones 2 and 3, and the rifes back up when the tidal outfalls close. Arun applies extra scrutiny to anything on the Pagham Beach Estate because of the coastal processes along it; it adopted an interim development framework for the estate in October 2021 and has been looking at a Coastal Change Management Area. It also states plainly that neither the council nor the Environment Agency has a duty to protect the coast from erosion or flooding.

Pagham Harbour itself is a Ramsar site, a Special Protection Area and a Site of Special Scientific Interest. It is not a National Landscape, so it does not switch off ordinary permitted development, but the larger home extension route is not available on SSSI land. Around Chichester Harbour, the National Landscape designation does reduce permitted development, and the Chichester Harbour Conservancy is consulted on applications there, with particular attention to dark skies — large glazing and rooflights can be questioned.

The ground under the coastal plain

The British Geological Survey's description of the Chichester and Bognor area explains why foundations here vary so much from one plot to the next:

• Brickearth — a fine wind-blown silt — covers much of the plain, usually under 2m thick.
• Raised beach gravels up to about 4m thick lie around Bognor Regis, Pagham and Selsey, with flint shingle along the present beaches.
• Head gravel, flint in clay, is thick along a buried former cliff line on the northern plain, which can mean the ground changes abruptly across a single site.
• Alluvium and tidal deposits — soft silty clays and organic muds — around Chichester and Pagham harbours and in the Arun valley at Littlehampton, along with made ground on reclaimed land.
• London Clay beneath much of the coast between Bognor and Pagham, which shrinks and swells with moisture and can be sulphate-bearing. Chalk lies deeper, and is exposed at Felpham.

In practice that means silt over gravel over clay, perched water where the silt sits on clay, and winter water tables close to the surface. Trenches may need pumping. Concrete may need to resist sulphates where tests show it. And a footing priced at one depth may need to go deeper once the trench is open.

Clay and trees: how deep the foundations go

On shrinkable clay, trees matter — the ones there now, and the ones recently felled, whose roots no longer draw water so the clay swells back. Foundation depths near trees follow published guidance (NHBC Chapter 4.2 and the LABC Warranty manual use the same framework), based on how shrinkable the clay is, the tree species and its distance:

Source: LABC Warranty Technical Manual, section 3: foundations.

Near trees those depths increase. Beyond about 1.5m, heave protection is needed against the sides of the foundation; beyond 2.5m the foundation has to be designed by a structural engineer, and with clay and trees that usually means piles and ground beams. That is why our quotations price foundations against a stated assumed depth, and the figure moves to match the depth actually reached once the trial holes are dug — up or down.

Rainwater and drainage

A new extension roof and a terrace send a lot of water somewhere. Soakaways only work where the ground drains, which on clay or with a high water table it often does not; infiltration tests settle it. Arun publishes supplementary requirements for surface water, including infiltration testing and groundwater monitoring. Where a soakaway will not work, the alternatives are attenuation and a controlled connection, agreed with Building Control. Shared drains at the back of older terraces and semis are often now public sewers, which brings in Southern Water's build-over rules — see extension costs.

Salt air and the materials that last

On a seafront elevation, the fixings decide how long a finish lasts. There is no single regulation that lists coastal specifications, but the good practice we follow on the front at Felpham, Aldwick, Middleton-on-Sea and Bognor Regis is:

• Stainless steel wall ties, restraint straps and fixings — marine-grade where exposed — rather than galvanised, which corrodes faster in salt air.
• Powder-coated aluminium windows, bifolds and lanterns with a coastal or "seaside" finish, and an understanding of the supplier's warranty conditions, which often require periodic washing down.
• Render systems with mesh and a silicone finish specified for severe exposure to wind-driven rain.
• Marine-grade sealants and pointing suited to the exposure.
• No untreated mild steel anywhere it can see the weather.

Get it wrong and you are back at the house in three years. On the Summerley Estate in Felpham, where we built a first-floor extension with a seaward glass balcony, the glass was lifted with one eye on the wind, and the specification was written for the frontage it faces.

Does building near the coast cost more?

Sometimes, and almost always below ground: deeper or engineered foundations, trench pumping, sulphate-resisting concrete, land drainage, and on the beach estates the logistics of weight limits and restricted hours. Above ground, coastal-rated fixings and finishes cost a little more than standard and save a great deal. Against that, wider roads and drives in the suburbs of Bognor and Felpham often make access cheaper than in a city centre. Our price guide bands are the same across our area; what changes is where in the band a job lands.

Questions people ask about building near the coast

Where these facts come from

• Environment Agency: flood risk assessment standing advice (updated May 2026).
• Environment Agency: Flood Map for Planning.
• Arun District Council: flood risk and the 2026 Strategic Flood Risk Assessment.
• British Geological Survey: Chichester and Bognor sheet explanation.
• LABC Warranty Technical Manual: foundations.
• Chichester Harbour Conservancy: planning.