Five rivers and a chalk aquifer under the garden
Ask anyone who has built at ground level in Salisbury what slowed the job down and water will come up. Salisbury sits where the Avon, Nadder and Bourne meet, and the council's own Central Area Framework says large parts of it lie in the flood plain. The Environment Agency runs flood warnings for The Friary and the city centre (Fisherton Street, Crane Street, Exeter Street, Friary Lane and the streets round them), for The Close and West Harnham along Harnham Road and St Nicholas Road, and for the Bourne through Milford Park and the Bishopdown side of Laverstock.
If your house is in Flood Zone 2 or 3, a planning application for an extension has to come with a flood risk assessment. For a householder job the assessment follows the Environment Agency's standing advice, which asks for the new floor to sit at least 600 millimetres above the estimated flood level, or for the room to be built to resist floodwater up to that height. Look your postcode up on the flood map for planning before anyone draws a plan.
Then there's the water you can't see. The council's groundwater strategy records that levels rose sharply round Salisbury in 2012, and that in the winter of 2013 to 2014 they reached the highest ever recorded, with flooding across the south of the county. The chalk holds it. The same strategy is wary of soakaways in susceptible areas, warns that soakaways in chalk can wash the ground out, and takes a dim view of basements. Ask where the roof water will go, and get a trial hole dug before the footings are priced. Near the rivers, as at Exeter Street, the ground is alluvium rather than chalk.
What the city's design guide will and won't accept
Salisbury's neighbourhood plan was made in March 2025. Its design policy says every development must have regard to the Salisbury Design and Advertising Guide, and adds that generic design approaches will not be tolerated. Wiltshire Council's own Wiltshire Design Guide was adopted as planning guidance in 2024 and sits alongside it.
On roofs the guide is blunt. Flat-roofed extensions will not normally be supported, and it calls them a crude and harmful addition to most buildings. A roof span wider than the original will not be permitted. The pitch should usually match the house, with the new ridge dropped and the wall set back so you can still tell which part was there first. Big additions that swamp the original house are out.
It isn't against modern work, though. A bold contemporary design can sit well on an old house, and more glass is fine if the proportions are right. It suggests windows set back at least 110 millimetres into the wall.
Owners of listed houses face more. Extensions should go to the back, a side addition should step back from both the front and rear walls, and uPVC conservatories or polycarbonate glazing will not be permitted on a listed building.
Choosing the right addition for a Salisbury house
Each type below links to a full guide.
Single-storey rear. The likeliest project on a Fisherton or St Paul's terrace and on the interwar semis at Pembroke Park. Outside the conservation areas a semi or terrace can go 3 metres beyond the original back wall without an application, and a detached house 4, or 6 and 8 with prior approval.
Planning. The Salisbury City conservation area takes in the old centre, and Milford Hill has its own. In either, side extensions, rear extensions over one storey and cladding all need permission. With more than 600 listed buildings in the city, check the listing before anything else.
Kitchen. Most of the budget on a Victorian terrace goes here. Spend it on the glazing and the layout rather than the last square metre.
Side return. The narrow passage beside a Windsor Street terrace is the classic spot. A side extension may only be single storey and up to half the house's width under permitted development, and none at all in a conservation area.
Open plan. Taking out the back wall means a steel beam sitting on new pads. On chalk with a high water table, let the engineer see the trial hole first.
Wrap-around. Always a planning application, because of the side leg. Better on the wider plots at Bishopdown or Paul's Dene than on a tight Victorian terrace, and the roof needs thought if flat roofs are off the table.
Two-storey. Under permitted development it can reach only 3 metres back and must stay 7 metres off the rear boundary. Most here need an application, and the design guide's set-back and lower ridge apply.
Ground-floor bedroom. A good answer for an older relative. In a flood zone the raised floor and a level threshold pull against each other, so draw the ramp at the same time. Wiltshire says the Avon phosphate rules do not generally touch householder extensions, but a self-contained annexe adding overnight accommodation is a different question.
Home office. An attached study shares the house's heating and broadband. A detached cabin at the bottom of a riverside garden needs its own flood check, since it may be on lower ground than the house.
Cost and value. No prices here. A flood assessment, deeper footings in wet chalk or clay tiles to match an old city roof each move the figure, so get three builders to look at the site.
Who signs it off
Wiltshire Council decides planning applications, and Salisbury City Council's planning committee gives its view on them. Building regulations go to Wiltshire Council's own building control team, whose office is in Chippenham. Book site visits with them. Before you start, look up past applications on your street. What the neighbours got through, and what they were refused, is the best guide to what you'll get.
Where these facts come from
- Salisbury Design and Advertising Guide (Salisbury City Council)
- Salisbury Neighbourhood Development Plan, made March 2025
- Wiltshire Council: Wiltshire Design Guide
- Wiltshire Council Groundwater Management Strategy 2016
- Salisbury Central Area Framework 2020 (Wiltshire Council)
- Environment Agency: The Friary and City Centre flood warning area
- Environment Agency: The Close and West Harnham flood warning area
- Environment Agency: River Bourne at Laverstock flood warning area
- Flood risk assessment standing advice (GOV.UK)
- Wiltshire Council: phosphorus and nitrogen mitigation
- Permitted development for householders, technical guidance (GOV.UK)
- Wiltshire Council: contact building control
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Questions people in Salisbury ask
We're on Harnham Road near the river. Can we still build out the back?
Usually yes, but check the flood map for planning first. In Flood Zone 2 or 3 the application needs a flood risk assessment, and the Environment Agency's standing advice expects the new floor 600 millimetres above the estimated flood level or built to resist water to that height.
Will the planners accept a flat-roofed kitchen extension in Salisbury?
It's an uphill argument. The city's design guide says flat-roofed extensions will not normally be supported, and the neighbourhood plan requires applications to have regard to that guide. A lean-to or a pitched roof matching the house is the safer route.
Can the rainwater from our new extension go into a soakaway?
Not automatically. Wiltshire Council's groundwater strategy discourages soakaways in areas prone to groundwater flooding unless an assessment shows they're safe, and notes that soakaways in chalk can erode the ground. Ask building control early.
Our cottage in the city centre is listed. Can we add a uPVC conservatory?
No. The city design guide says uPVC conservatories and polycarbonate glazing will not be permitted on listed buildings. You'd need listed building consent for any extension, and it should normally sit at the back.