Flood Risk Assessment in Guildford

A flood risk assessment in Guildford starts with the River Wey, which runs through the middle of the town, past Millmead, the Friary and Walnut Tree Close, in the gap it has cut through the chalk of the North Downs. The borough's flood evidence counts part of that valley floor as functional floodplain even where buildings already stand on it, and there are no formal flood defences. Rain off the steep streets and groundwater in the chalk add to the river; the property flooding recorded in 2024 happened away from it. Applicants, buyers, lenders and insurers commission our flood risk assessments in Guildford, and the same evidence feeds the sequential and exception tests and surface water drainage strategies we write for the town.

Guildford here means the town as the ONS maps its built-up area: the centre around the High Street and North Street, Stoke and Bellfields, Slyfield and Burpham to the north-east, Merrow to the east, and Onslow Village, Guildford Park, Westborough, Park Barn, Woodbridge Hill and Stoughton to the west and north-west, across the GU1, GU2 and GU4 postcode districts. Shalford, Artington and Jacobs Well lie just outside the outline, though the same councils and the same Wey evidence apply to them.

In the early hours of Christmas Day 2013 the Wey at the Guildford gauge reached 3.72 m, nearly a metre above the 2.80 m at which the Environment Agency says property flooding becomes possible. The station's record, set on 7 November 2000, is higher still at 3.88 m.

Ring 01293 214444 or email enquiries@unda.co.uk to talk through a Guildford address with one of our flood risk consultants.

Where the Wey cuts through the North Downs

Guildford grew up at a ford where the Wey cuts through the North Downs ridge, with The Mount to the west and Pewley Down to the east rising about 150 m, as Surrey County Council's 2024 Section 19 report on the town centre describes it. That setting gives the town four sources of flooding, and the Level 1 Strategic Flood Risk Assessment maps each of them:

• The River Wey. A main river, managed by the Environment Agency. South and north of the centre its 1 in 20 year flood spreads across open meadows such as Shalford Park and Woolgers Wood, but just north of the Rowing Club the river and its floodplain become urban. The SFRA puts low-lying property east of Walnut Tree Close and at Woodbridge Meadows at high risk, and in the 1 in 100 year flood much of the industrial estate north of the A25 and the homes north of the river along Stoughton Road. Our River Wey page follows the river from its two sources to the Thames.
• The Wey and Godalming Navigations. The Wey Navigation begins at Millmead Weir and runs beside the river to the Thames, and the Godalming Navigation runs upstream. Both are owned by the National Trust and share the valley floor and, in places, the river channel itself, so a site beside the towpath answers to the river's flood levels.
• Tributaries above the town. The Tillingbourne joins the Wey at Shalford, and Cranleigh Waters joins it at Peasmarsh, so rain over the Surrey Hills and the Weald reaches the town down two tributaries as well as the main river.
• Surface water. The SFRA picks out high-risk flow routes along roads and small watercourses in Merrow, Burpham, Woodbridge Hill and the town centre. The 2013 surface water management plan names Glendale Drive, Gosden Hill Road and Merrow Lane in Burpham, where blocked culvert inlets cause the flooding. Our note on surface water flooding explains why a Flood Zone 1 plot can still be at risk.
• Groundwater and sewers. Chalk and greensand underlie much of the urban area, especially the south of the town centre. Thames Water's register held 64 records of sewer flooding inside property and 208 outside across the borough up to September 2014, mostly in postcodes beside the rivers.

Fluvial flood risk from the Wey shapes most assessments in the centre, because the Environment Agency is consulted on development in Flood Zones 2 and 3 and will object where the river evidence is weak. Our note on river flooding covers how it is assessed.

What the Guildford gauge on the Wey has recorded

The Environment Agency's River Wey at Guildford gauge sits in the town centre a short way downstream of Millmead and has measured levels since 1995. Its published thresholds give a working scale for anyone with a riverside site in the town:

1. 1.10 m to 2.60 m. The normal range.
2. Above 2.60 m. Low-lying land may flood and flood alerts can go out.
3. Above 2.80 m. Property flooding is possible.
4. 3.71 m. The level of a past event that flooded property in Guildford.
5. 3.88 m on 7 November 2000. The highest level the station has recorded.

The River Wey at Guildford flood warning area covers the river through the town, Riverside Park Nature Reserve and Jacobs Well. Our live river levels map shows the latest readings and any warning in force. A gauge reading is not a design level, though: an assessment works from modelled flood levels with an allowance for climate change, and the 2000 and 2013 readings are what those modelled levels get checked against.

Screening an address from Stoughton to Merrow

Enter a postcode in the map below to screen a Guildford address against Environment Agency data for rivers, surface water and reservoirs, or open our full flood risk map for anywhere in England. The coral line is the ONS built-up area, which follows the edge of the buildings, so the town centre, Burpham, Merrow and Park Barn fall inside it while the open meadows along the Wey beyond the built edge fall outside, even where they flood.

Guildford flooding from 1968 to 2024

The SFRA lists large Wey floods in 1900, 1928, 1968, 1979, 1990, 2000, the winter of 2002 to 2003, 2006, 2007 and the winter of 2013 to 2014, and records 184 flood incidents reported along the Wey in the GU1 and GU2 postcodes between 2000 and 2014. These are the events with the clearest detail:

1. 15 to 16 September 1968. After 3.75 inches of rain on the Sunday, measured at Stoughton, the Wey burst its banks through the centre overnight. The bottom of the High Street stood under about six feet of water, up to 300 people were evacuated, and plaques on St Nicolas Church still mark the level, as the Guildford Dragon recalled on the flood's fiftieth anniversary.
2. 7 November 2000. The Guildford gauge reached 3.88 m, its record.
3. 24 and 25 December 2013. The Environment Agency issued a flood alert at 10.11am on Christmas Eve, the river stood at 3.23 m by 12.30pm and the gauge peaked at 3.72 m at 3.15am on Christmas Day. Millmead car park went several feet under, the Yvonne Arnaud Theatre cancelled its Christmas Eve performance, Mary Road took about a foot of water and a family was forced out of a Walnut Tree Close home on Christmas Day (Guildford Dragon).
4. 7 February 2014. The Wey came out again at Millmead, closing the car park and the road down from the High Street (Guildford Dragon).
5. December 2019 and February 2020. The Environment Agency put up temporary barriers when flows were high; its scheme summary gives a possible deployment area around Mary Road and William Road.
6. 1 August, 8 September and 14 October 2024. Fifteen properties and one commercial building flooded, mostly outside the flood zones along surface water routes and mostly south of York Road. Surrey County Council's Section 19 report blames steep streets that send water past the gullies, blocked or buried soakaways, private drains discharging onto the road and a watercourse blocked by fly-tipping, after a year of exceptional rainfall and groundwater.

There are no formal flood defences in the borough. After the winter 2013 flooding the borough council began a project to fit property-level protection to about 35 properties in William Road, Mary Road, Leas Road and Walnut Tree Close, the streets that flooded that winter. A permanent scheme is being worked up by the Environment Agency with the borough and county councils, and its Outline Business Case went to government in June 2026; our article on the Guildford Flood Alleviation Scheme sets out the proposals. Until it is built, an assessment works from today's flood levels.

Developed and undeveloped Zone 3b in the town centre

Guildford's SFRA draws its functional floodplain in two parts. It takes the modelled 1 in 20 year outline as Flood Zone 3b and then splits it: the footprints of existing buildings inside it are "developed" 3b, and everything else, including surface car parks and gardens, is "undeveloped" 3b. Policy P4 of the Local Plan treats the two very differently:

Where no 1 in 20 year outline has been modelled, the SFRA says Flood Zone 3 should stand in for 3b or the extent should be agreed with the Environment Agency. The footprint rule pushes town-centre schemes towards undercrofts, colonnades and floodable voids rather than level-for-level compensatory storage, because a tight plot seldom has room to lower ground elsewhere. On one mixed-use redevelopment in central Guildford with developed 3b at the rear, our assessment used the latest Wey modelling and a ground-floor colonnade so the flats above overhang without enlarging the footprint, and the Environment Agency withdrew its objection.

Which Guildford policies does a flood risk assessment answer?

Guildford Borough Council decides planning applications in the town using two Local Plan documents, and Surrey County Council reviews the surface water drainage on major schemes as Lead Local Flood Authority. The flood policies an assessment has to meet are these:

• Policy P4 (Local Plan: Strategy and Sites, adopted 25 April 2019). Development in Flood Zones 2 and 3, including developed 3b, is permitted only where the use suits the risk and the sequential and exception tests are passed. A site-specific assessment has to show the scheme and its access will be safe for its lifetime, allowing for climate change, without raising risk elsewhere; resistance and resilience measures must be built in, flood warning and evacuation plans put in place where relevant, and the drainage sized for a 1 in 100 year storm plus climate change. The supporting text asks for a groundwater assessment wherever a site may be at risk from groundwater (Local Plan: Strategy and Sites).
• Policy P10 (Development Management Policies, adopted 22 March 2023). Keeps an undeveloped buffer of at least 10 m from the top of the bank of a main river such as the Wey, protected during building and managed afterwards, and rules out new culverts (Development Management Policies).
• Policy P11 (Development Management Policies). Greenfield sites must match greenfield runoff rates and volumes; brownfield sites must get as close to them as reasonably practicable and never exceed the site's existing runoff. Permeable surfaces are to be maximised, boreholes avoided except for clean roof water, and major and at-risk schemes must follow the discharge hierarchy and the county council's guidance.
• The SFRA. A Level 1 study by Capita (Volume 1, July 2016) and a Level 2 study of the allocated sites (May 2016). Both still underpin the adopted plan.
• A new Local Plan. Preparation began under the 2026 plan-making rules: a scoping consultation closed on 14 October 2026, the first gateway is due on 30 October 2026 and the council's timetable programmes adoption for 30 April 2029.
• West Surrey Council. From 1 April 2027 a single West Surrey Council replaces Guildford Borough Council and its neighbours in the west of the county and takes over Surrey County Council's services there. The existing councils carry on as normal until then; our article on local government reorganisation explains what it means for applications in progress.

Nationally, a Guildford application is judged against chapter 18 of the August 2026 NPPF: policy F4 sets when a site-specific assessment is needed, F5 and F6 the sequential and exception tests, and F8 expects every development to follow the 2025 national standards for sustainable drainage. The council's own validation checklist still quotes the 2019 version of the NPPF, so its paragraph references are out of date even though its triggers still hold.

Bedford Road, Walnut Tree Close and Weyside: the allocated sites

Several of the town's allocations sit on or beside the Wey floodplain. The Level 2 SFRA tested them in 2016, and the Local Plan then wrote flood requirements into each one:

Weyside depends on moving the sewage works at Slyfield. Thames Water says the new works will be commissioned from early 2027 and fully running by autumn 2027. A windfall site that is not allocated, or a use the allocation does not cover, has to pass the sequential test on its own, with Policy P4's reasoned justification asking it to show it is preferable to the sites already allocated.

Chalk, greensand and soakaways under the town

The ground decides whether rain on a Guildford site can soak away or has to be stored and released slowly, and it changes within a few streets of the centre:

• South of York Road. Mostly free-draining chalk, according to the Section 19 report, yet this is where the 2024 flooding was worst, because steep, built-up streets carry water faster than it can soak in.
• North of York Road. Ground of highly variable permeability, where an infiltration test on the plot is the only reliable guide.
• Groundwater protection. About 30% of the borough sits on principal aquifers and it holds 14 Source Protection Zones. The Jewsons allocation at Walnut Tree Close lies in an inner zone (SPZ1), where Policy P11 asks for a hydrogeological risk assessment before anything but clean roof water goes into the ground.
• The 1 m rule. Policy P11 needs at least 1 m between the base of a soakaway and the highest likely groundwater level. Groundwater in the Guildford area was exceptionally high in September 2024, so a groundwater level read in a dry spell can understate the true maximum. Tests follow BRE Digest 365; our BRE 365 infiltration rate calculator works through the results and the ground conditions checker gives a first view of the geology.
• County standards. Surrey County Council has assessed every new consultation against the 2025 national standards for sustainable drainage since 6 October 2025, with separate design guidance for major and non-major schemes and chargeable pre-application advice.

Guildford proposals and the assessment each one needs

Policy P4, the council's validation checklist and chapter 18 of the NPPF together decide when a Guildford application needs a flood risk assessment. This is how they apply to the kinds of proposal the town sees:

Our guide to when a flood risk assessment is needed covers the national triggers, and our note on conveyancing flood reports explains why a search result cannot go in with an application.

What our consultants prepare for sites along the Wey and up the valley sides

A buyer in Mary Road, a householder in Merrow and the developer of a town-centre allocation each need a different report, and the Wey, the chalk and the steep streets turn up in most of them. Every report is written by chartered consultants and signed off by a senior consultant.

• Applications in the town-centre floodplain. Homes, conversions and commercial schemes beside the Wey need a flood risk assessment for planning that answers Policy P4, the SFRA's Zone 3b mapping and the Environment Agency.
• River, rain, chalk and sewers together. A flood risk assessment in Guildford weighs the Wey, the surface water routes south of York Road, groundwater in the chalk and the public sewers as one picture.
• Buying in a warning-area street. Mary Road, William Road and Walnut Tree Close show up in searches, and a flood risk survey for property purchase looks at the house rather than the postcode.
• Homes in Zones 2 and 3. Windfall housing in the Wey flood zones, and allocations where Policy P4 calls for it, need the sequential and exception test.
• Runoff on every scheme. Policy P11 asks every proposal to show its runoff will not get worse, so most Guildford applications need drainage strategy work of some kind.
• Chalk on one side of York Road, mixed ground on the other. A surface water drainage strategy tests infiltration against the 1 m groundwater rule and falls back to storage and a controlled discharge where the ground will not take the water.
• Conditions on a permission. Where the county council's standard drainage condition is attached, discharge of drainage conditions supplies the details and maintenance plan needed to clear it.
• Confirming the 1 in 20 year outline. The SFRA defines Zone 3b from modelled outlines, and flood modelling of the Wey can confirm the levels and extents for a single plot.
• Warnings and a dry route out. Policy P4 asks for flood warning and evacuation plans where relevant, and a flood warning and evacuation plan ties a riverside scheme to the River Wey at Guildford warning area.
• High water tables in the chalk. The Local Plan expects a groundwater assessment wherever a site may be at risk, and a groundwater flood risk assessment covers basements and soakaways after a wet year like 2024.
• Cover after Christmas 2013. Homes in Walnut Tree Close and Mary Road took water that winter, and a flood risk assessment for insurance gives an insurer evidence for one address.
• Theatres, shops and offices by the river. The Yvonne Arnaud lost its Christmas Eve 2013 performance to the flood, and a business continuity flood risk assessment plans for a day like that.
• Offices at Walnut Tree Close and workspace at Weyside. The A8 office site lies almost wholly in Zone 3b and Weyside includes light industrial space, so a BREEAM flood risk assessment covers the Pol 03 credits for new commercial buildings.
• Before an offer on a riverside plot. With the footprint rule in developed 3b, a flood risk feasibility study shows early whether the floor area a buyer needs will fit.

Reporting flooding and getting warnings in Guildford

Where floodwater puts anyone in danger, call 999 and ask for the fire and rescue service; call 105 if the power goes off. For everything else, each organisation below looks after one part of Guildford's flood risk:

The county as a whole is covered on our Surrey page. Upstream on the Wey is Godalming, up Cranleigh Waters is Cranleigh, and downstream the river meets the Thames at Weybridge. Every other town and county we cover is in the list of areas.

Guildford flood risk questions, from Millmead to Merrow

From Millmead to Merrow, Unda Consulting Limited's chartered consultants assess flood risk and drainage on Guildford sites, and a senior consultant checks and signs off every report; Unda is a CIWEM Business Partner. Call 01293 214444 or email enquiries@unda.co.uk.