Many Cornish homes sit on generous plots, and some owners wonder whether part of the garden could take a new house. It might be a home for a family member, a self-build for yourself, or a plot to sell. Building a new dwelling in a garden, sometimes called plot severance or infill development, can work well, but it involves more checks than an extension or outbuilding.
This guide explains the main planning, legal and practical points to consider in Cornwall before you spend money on drawings. It is general guidance only, so always take advice for your own site. If you want a building that stays part of your existing home, such as space for a relative, our annexe guide covers that route instead.
Is a house in your garden likely to be acceptable?
A new house always needs full planning permission. It cannot be built under permitted development. Cornwall Council will assess it against the Cornwall Local Plan, any neighbourhood plan for your parish and national policy. Key questions usually include:
Location: is the site within or on the edge of an existing town or village? In smaller settlements, the Local Plan generally supports infill, meaning filling a small gap in an otherwise continuous built frontage, and some rounding off. New homes in open countryside are much more tightly controlled.
Character: would a new house fit the pattern of the area, or would it look cramped compared with neighbouring plots?
Neighbours: overlooking, loss of light and outlook, and noise for nearby homes, including your own.
Amenity space: both the existing house and the new one need reasonable gardens and parking.
Designations: sites in a conservation area, the Cornwall National Landscape or near listed buildings face extra scrutiny. Some neighbourhood plans in Cornwall also include principal residence policies that restrict new homes from being used as second homes.
Our guides to planning permission in Cornwall and building in an AONB or conservation area explain more. Cornwall Council offers pre-application advice, which can be worth using before a full application.
Access and parking
A new house needs safe access. Highways officers will usually look at whether a new or shared driveway can provide adequate visibility onto the road, room to turn and enough parking. Narrow Cornish lanes and high hedges can make this a deciding issue. Sharing the existing driveway is sometimes possible but may need widening. Our guide to dropped kerbs and new driveways covers vehicle crossings.
Drainage, services and ground conditions
Before committing to a design, check how the new house will be served:
Foul drainage: connecting to a public sewer is normally preferred. Where there is none, a package treatment plant or other off-mains system may be needed, with suitable ground for discharge. See our guide to septic tanks, drainage and soakaways.
Surface water: soakaways or other sustainable drainage are usually expected, which depends on how well the ground drains.
Water and electricity: new connections can take time, so contact providers early.
Ground: old mine workings, made ground or slopes can affect foundation design. Our guides to building on old mining land and groundworks and foundations explain more.
Radon: parts of Cornwall need radon protection in new homes; see our radon guide.
Trees, wildlife and biodiversity net gain
Established gardens often contain trees, hedges and wildlife. Trees covered by a Tree Preservation Order or in a conservation area are protected; see our guide to building near trees. Ecological surveys may be needed, particularly if bats are likely to be present.
Most new homes in England now also need to deliver biodiversity net gain, meaning the development should leave the site's habitats in a measurably better state. There are some exemptions, including for certain self-build and custom-build homes and very small areas of habitat loss. Check how the rules apply to your site early, as they can affect the layout.
Community Infrastructure Levy
Cornwall Council charges the Community Infrastructure Levy (CIL) on new homes, with rates that vary by area. Self-builders can often claim an exemption, but the claim must normally be made and approved before work starts on site. Missing this step can be expensive, so check the current rules with Cornwall Council as soon as you have permission.
Legal and financial checks
Before you apply, it is worth speaking to a solicitor about:
Title and covenants: some properties have restrictive covenants preventing additional houses, or rights of way across the land.
Mortgage: if your home is mortgaged, your lender will normally need to agree to part of the land being separated.
Value: reducing the garden may affect the value of your existing house, so a local estate agent's view can help.
Tax: selling a plot or a new house can have tax implications, so take professional advice.
Design, Building Regulations and the build
Once the principle looks workable, a designer can prepare drawings that suit the plot and the street. Our guide to choosing an architect or designer explains the options. A new home must meet Building Regulations, including energy efficiency standards, and lenders and buyers usually expect a structural warranty. Our guides to self-build homes in Cornwall, timber frame vs masonry construction and eco home building cover the build itself.
Frequently asked questions
Do I need planning permission to build a house in my garden?
Yes. A new, separate dwelling always needs full planning permission. It cannot be built under permitted development.
How big does my garden need to be?
There is no fixed minimum. What matters is whether the new house fits the character of the area, leaves both homes with adequate space and parking, and does not harm neighbours.
Can I build a house in my garden for a family member?
Yes, but if it is a separate home it is assessed like any other new dwelling. An annexe that stays part of your home is a different route with different rules.
Thinking about a new home on your plot? Talk to us
Cornish Building Company builds new homes across Cornwall, working alongside architects and engineers. See our new builds, groundworks and foundations and project management and QS services, and our areas we cover. Call us on 01208 816991 or contact us to talk through your project.
