Look at the dyke before you look at the drawings
Plenty of Boston gardens back onto a drain. The town grew on fen that was dried out in the eighteenth century, and the channels that did the job are still there, many of them run by an internal drainage board rather than the council. That matters more for an extension than almost anything else on this page.
The Witham Fourth District board covers the land from Boston up towards Coningsby and Wainfleet. Its Byelaw 10 says nobody may put up a building or structure, temporary or permanent, within 9 metres of the edge of a watercourse without the board's consent. The distance runs from the landward foot of the bank, or from the top of the slope where there's no bank. The Black Sluice board, which looks after the South Forty Foot Drain, has the same 9 metre rule and adds fences to the list.
This sits outside planning altogether. An extension can be permitted development and still need the board's say-so, and piping roof water into the dyke needs its own consent too. Ring the right board before you pay for plans.
The flood paperwork for a floodplain town
The Institution of Civil Engineers describes Boston as lying entirely within a floodplain. The 2013 surge put water into more than 800 properties, and the Barrier has guarded the Haven since 2020. None of that stops you building, but a planning application here will usually come with a flood risk assessment.
For a house extension it's lighter than it sounds. The Environment Agency treats anything up to 250 square metres of floor as a minor extension, and these skip the sequential test that new houses face. You follow its standing advice instead. That advice recommends setting the new floor 600mm above the estimated flood level, or 300mm where that level is known with confidence. Where a floor can't go that high, it wants flood resistant and resilient construction instead, with plaster, sockets and finishes chosen to survive water.
Boston Borough Council publishes a Flood Risk for Householders note alongside its application guidance. Download it before your designer draws the floor, because the floor height drives the threshold, the step down from the old kitchen and the roof line.
What suits a Boston house
Detached houses and semis far outnumber terraces across the borough, and bungalows are everywhere from Carlton Road to the northern estates. Here's how each type tends to fit, roughly in the order we'd raise them.
Single storey at the back. The everyday Boston job. Outside a conservation area a semi or terrace can go 3 metres back under permitted development and a detached house 4, or 6 and 8 metres with prior approval, so long as no dyke sits within 9 metres.
Downstairs bedroom. Bungalows make this easy because everything is already on one level. Talk flood levels first, as a bedroom is the room you least want close to the ground in a floodplain, so set the floor level with care.
Kitchen. Most semis built between the wars or after 1945 end up with this brief. If the new floor has to sit higher than the old one, plan the step or the ramp into the layout rather than finding it on site.
Wrap-around. Worth a look on a detached house with space down one side. In a conservation area the side leg rules it out of permitted development, and anywhere else it is usually an application.
Open plan. Opening a back wall means a steel beam and an engineer's calculations for building control at Municipal Buildings. Get those before you ask builders to price it.
Side return. Suits the narrow Victorian terraces round Pen Street and Grove Street and the railway streets off Tower Street. Under national rules it's single storey only and no wider than half the house.
Two storey. Permitted development wants 7 metres to the back boundary, which many terraced gardens can't give, so expect an application and with it the flood questions.
Home office. An attached room avoids a separate cabin at the end of the garden, which is often exactly where the dyke runs.
Planning. Three separate checks here. Planning or permitted development comes first, then drainage board consent, then Building Regulations. Missing the middle one is the Boston mistake.
Cost. We don't print a Boston figure. Raised floors, resilient finishes and deeper foundations on drained fen all move the price, and only a site visit shows which apply.
Skirbeck, Spilsby Road and the old town
Three of the borough's 11 conservation areas take in parts of the town: the centre around the Market Place, Skirbeck around St Nicholas church, and Spilsby Road with its Victorian villas. National rules strip back permitted development in all three. No side extensions, no rear extensions over one storey, and no cladding of any outside wall, whether in timber, tile or render.
A single storey at the back can still be permitted development. Anything else means an application, and the council's conservation team (shared through the South and East Lincolnshire partnership) will want brick, bond and roof to match the house. A listed house needs listed building consent as well. The council only gives pre-application advice through its own written protocol, so read that before booking a meeting.
Where these facts come from
- Witham Fourth District IDB: byelaws
- Witham Fourth District IDB: development consent guidance (2023)
- Black Sluice IDB: byelaws
- Environment Agency: flood risk standing advice (minor extensions)
- Environment Agency: flood risk assessment in flood zones 2 and 3
- GOV.UK: permitted development rights for householders, technical guidance
- Boston Borough Council: submitting a planning application
- Boston Borough Council: conservation areas
- Boston Borough Council: pre-application advice
- Institution of Civil Engineers: Boston Barrier
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Questions people in Boston ask
There's a dyke behind our garden. Can we still extend?
Usually, but measure first. Both the Witham Fourth and Black Sluice boards need consent for any building within 9 metres of a watercourse's bank. Ask the board early. Its answer can shape the depth of the extension more than the planners do.
Do we need a flood risk assessment if the extension is permitted development?
No. Permitted development doesn't go through planning, so there's nothing to attach it to. Building Regulations still apply, and it's worth asking your builder about raised sockets and resilient plaster anyway.
We live in the Skirbeck conservation area. Can we build out to the side?
Not under permitted development. Side extensions are excluded in conservation areas, so you'd need planning permission, and the design will need to sit quietly beside the house.
Can a Boston bungalow take a two storey extension?
It can be done, but you're changing the house as much as extending it. On a bungalow estate we'd usually suggest a single storey addition, or a loft room, before anything that adds a full upper floor.