Granny annexe cost: the factors that shape your budget

Size and layout

Floor area is the starting point, but layout matters almost as much. An open-plan studio with a separate shower room uses fewer internal walls, doors and finishes than a one-bedroom layout with a hallway. Every extra room adds partitions, a door, lighting, heating and decoration.

Ceiling height and roof shape also play a part. A pitched roof gives more headroom and often sits better beside a house, but it uses more structure and covering than a simple flat roof.

How big can a granny annexe be?

There is no single national maximum. Where an annexe is built under permitted development as an outbuilding, height limits apply and it cannot be used as a separate bedroom-based living space, so most lived-in annexes need planning permission. The council then looks at size in relation to the main house, the garden and the neighbours, and generally expects the annexe to stay ancillary to the main home rather than becoming a separate dwelling.

In practice, the size you can build is a planning question first and a cost question second, so it is worth agreeing the footprint with the council before detailed pricing.

Foundations and ground conditions

Ground conditions are one of the biggest unknowns. Sloping gardens, soft ground, nearby trees and drains all affect the type and depth of foundation. A level, accessible plot with firm ground is the simplest case; a garden that needs retaining, stepping or deeper foundations will cost more before anything is built above ground.

Services: drainage, water, power and heating

Connecting the annexe to the main house's services is often underestimated. Foul drainage for a kitchen and shower room may need a long run or a pump if the annexe sits lower than the existing drains. Water, electricity and data need trenching, and the consumer unit or supply may need upgrading.

Heating choice affects both build cost and running cost. Electric systems are simpler to install, while extending a wet heating system or fitting a heat pump involves more work up front.

• Distance from the house to the annexe
• Gravity drainage or a pumped connection
• Electrical supply capacity
• Heating system type

Construction method and insulation

Annexes are commonly built in masonry or timber frame. Both can meet building regulations; the choice affects programme, foundations and finishes. Because an annexe is a home lived in all year, it needs the insulation, ventilation and fire safety of a habitable building, which is a key difference from a basic garden room.

Kitchen, bathroom and accessibility

The kitchen and shower room are usually the most expensive rooms per square metre. Specification ranges widely, from a compact kitchenette to a full kitchen. If the annexe is for an older relative, level access, a walk-in shower, wider doors and grab rails are worth planning in at the start, because adding them later costs more.

To talk through a build, see our garden rooms and annexes service.

Fees and approvals

Allow for drawings, a planning application, building regulations approval and, where needed, structural design. These sit alongside the building cost and are easy to leave out of an early budget.

Common questions