Your neighbour's window decides a lot
On a Cambridge terrace the house next door is a few feet from your back wall, so officers start with the neighbour. Policy 58 of the Cambridge Local Plan 2018 allows alterations that "do not unacceptably overlook, overshadow or visually dominate neighbouring properties". Planning reports turn that into a drawing test. They take a 45-degree line from the nearest window next door, following the BRE daylight guidelines, and check whether the new wall crosses it.
Closer cases bring in the BRE benchmark of two hours of sunlight on 21 March. Ask your designer to draw the 45-degree line on the first sketch. If the new roof cuts through it, a lower eaves or a pitch that falls away from the boundary is often enough to pull it back.
Two-storey side additions face a different test. Officers look at whether the gap left to the next house is wide enough to avoid what they call the terracing effect, where a row of semis starts to read as one continuous block. There is no fixed set-back figure, so look at the gaps the neighbours have kept.
Which addition suits which Cambridge house
Side return. Made for the gault brick terraces of Romsey, Petersfield and Newnham Croft, where a narrow passage runs beside the back wing. In a conservation area, building out beyond that side wall needs planning permission, even at one storey.
Kitchen. Most terrace owners here want a kitchen that opens onto the garden. The Mill Road appraisal describes long, thin back gardens, so there is usually depth to spare. Spend the effort on a rooflight over the old middle room, which is often the darkest space in the house.
Single-storey rear. Outside a conservation area you can usually go 3 metres past the original back wall of a terrace or semi (4 if detached) without an application. The prior approval route to 6 or 8 metres is closed inside one, and on a terrace that depth rarely clears the 45-degree line anyway.
Open plan. Taking out the wall between the back wing and the old dining room puts a steel beam over the opening. On Gault clay the engineer will care as much about the pads under the posts as the beam itself.
Wrap-around. Filling the side passage and the rear at once gives the biggest kitchen, but on a terrace it lands right on the neighbour's boundary. Expect a planning application and a careful light study.
Two storey. Better suited to the semis on the post-war estates than the Victorian terraces. Without an application it can only reach 3 metres and must stay 7 metres off the rear boundary. In a conservation area it always needs permission.
Downstairs bedroom. Easier on the post-war houses and bungalows of the Arbury estate than on a narrow terrace. Put a shower room next to it and keep the threshold level with the garden.
Home office. If you work from home every day, a room built onto the house is warmer and easier to wire than a cabin at the end of the garden. Light footings near a big tree are worth an engineer's opinion first.
Planning. Check three things first. Is the house in one of the city's 18 conservation areas? Is it on an estate whose permission took away householder rights? Is it a flat? Flats have no householder permitted development at all.
Cost. We don't quote a Cambridge figure. A light study or a flood check near a brook adds cost. So do foundations on clay and carrying everything down a back alley. Three builders pricing the same drawings will show you the real spread.
When the brooks and drains overflow
River flooding in Cambridge is less about the Cam in the centre than about the smaller watercourses. The council's 2006 flood risk study explains that Bin Brook was made a main river mainly because of the risk along Grange Road, and that the May 1978 flood reached 22 houses in the Gough Way area. October 2001 was the worst event in recent memory and led to the Riverside flood defences.
Surface water is the wider risk. The Cambridge and Milton surface water plan lists hotspots including King's Hedges and Arbury, Nuffield Road, Chesterton High Street and St Thomas Square in Cherry Hinton. If the government flood map puts your house in Zone 2 or 3 and you need an application, a flood risk assessment goes in with it. For an addition of no more than 250 square metres the Environment Agency's standing advice for minor extensions covers it.
The council's 2006 study describes free-draining gravel terraces sitting over poorly draining clay, and the Cam valley is underlain by Gault clay and chalk. Ask the engineer about nearby trees before settling on a foundation depth, and get building control from 3C Shared Services to look at the trench.
Where the national allowances shrink
Large parts of Victorian and Edwardian Cambridge sit in conservation areas, from Mill Road and Romsey to De Freville, Newnham Croft and Chesterton. There, any extension past a side wall, any rear addition of more than one storey and any new cladding needs permission. The Chesterton appraisal criticises large rear and side additions and an incongruous flat roof, so a design that respects the old house will fare better.
Trees count too. Six weeks' notice to Greater Cambridge Shared Planning is needed before work on any tree in a conservation area, and Chesterton alone has several Tree Preservation Orders.
Then there is Accordia, off Brooklands Avenue. Its Article 4 direction from 2014 removes the right to enclose a recessed entrance or an open terrace, to add window openings and to reclad in a different material. On the newer estates, read your own decision notice, since a condition can remove rights an older street still has.
Where these facts come from
- Cambridge Local Plan 2018
- Cambridge City Council: Mill Road area appraisal (2011)
- Greater Cambridge Shared Planning: Chesterton and Ferry Lane conservation area appraisal (2022)
- Greater Cambridge Shared Planning: Article 4 direction, Accordia
- Greater Cambridge Shared Planning: tree works in a conservation area
- Cambridge City Council: Strategic Flood Risk Assessment (2006)
- Cambridge and Milton Surface Water Management Plan (2011)
- GOV.UK: flood risk assessment standing advice
- GOV.UK: Permitted development rights for householders, technical guidance
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Questions people in Cambridge ask
Our terrace north of Mill Road opens straight onto the pavement. How does a builder get materials to the back?
Through the house, or down the back alley if your row has one. The Mill Road appraisal notes that some gardens are reached by a narrow alley. Ask every builder how they plan to move spoil and steel before you compare quotes.
Next door says our rear extension will take their light. How will the council decide?
Officers usually draw a 45-degree line from the neighbour's nearest window, following the BRE guidelines, and check whether your extension crosses it. In closer cases they look at sunlight to the garden as well. Have your designer show the line on the plans.
We live near Bin Brook. Will we need a flood assessment for a kitchen extension?
Only if the house falls in Flood Zone 2 or 3 and the work needs planning permission. Check the government flood map for your address. If it does, the Environment Agency's minor extensions advice sets out what to include.
We're on the Accordia estate. Can we glaze in our open terrace?
Not as permitted development. The 2014 Article 4 direction removes the right to enclose an open terrace or recessed entrance, so you need a planning application.