Extending a detached house
Reviewed October 2026 · Not legal, planning or financial advice
Detached houses usually have the most options: space on more than one side, a larger exempt upper-floor allowance (20m²) and fewer party-wall issues.
Options
- Rear single-storey or two-storey extension.
- Side extension, single or two storey, where the plot allows.
- Wraparound for a large open-plan ground floor.
Proportion and value
On a detached house the risk is not space but proportion — an extension that dwarfs the original house or doesn’t connect well internally. Spending on layout and flow usually returns more than spending on floor area.
Rural houses
One-off rural houses often have septic tanks or wastewater treatment systems and percolation areas. An extension must not compromise them, and adding bedrooms can affect the system’s capacity — check before design.
Regulations and their interpretation change. This guide explains the position as we understand it in October 2026. Confirm what applies to your house with an architect, planning consultant or your local authority before relying on it.