Two-storey extension guides
Costs, rules and practical advice, checked against official sources. Where the rules differ between England, Wales, Scotland and Northern Ireland, we say so.
How much does a two-storey extension cost in 2026?
A two-storey extension typically costs £2,000–£2,700 per m² of new floor area in 2026, including VAT, which puts a 28 m² rear extension at about £56,000–£75,500.
Read the guideDo I need planning permission for a two-storey extension?
Often yes: a two-storey extension needs planning permission unless it meets the strict permitted development limits for your part of the UK, which in all four nations can allow some two-storey rear extensions.
Read the guideCan I build a two-storey extension under permitted development in England?
Yes, in England a two-storey rear extension to a house can be permitted development if it extends no more than 3 m beyond the rear wall of the original house and stays at least 7 m from the boundary opposite that wall.
Read the guideWhat are the rules for two-storey extensions in Wales?
In Wales, a two-storey rear or side extension can be built without planning permission if it meets the Welsh permitted development rules, which differ from England's.
Read the guideWhat are the rules for two-storey extensions in Scotland?
In Scotland, a two-storey extension can be permitted development under Class 1B only if no part of it is within 10 m of any boundary, it is no higher than the existing house, it does not front a road, and the house is not a flat or in a conservation area.
Read the guideWhat are the rules for two-storey extensions in Northern Ireland?
In Northern Ireland, a two-storey rear extension can be permitted development if it extends no more than 3 m beyond the rear wall of the original house and stays at least 7 m from the boundary opposite.
Read the guideWhat building regulations apply to a two-storey extension?
In England, a two-storey extension must meet the Building Regulations 2010, chiefly Part A (structure), Part B (fire safety), Part L (energy), Part F (ventilation), Part P (electrics) and Part H (drainage), whether or not it needs planning permission.
Read the guideDo I need a party wall agreement for a two-storey extension?
In England and Wales, you need to serve notice under the Party Wall Act if your two-storey extension involves building a new wall on the boundary line, cutting into or raising a shared wall, or digging foundations close to a neighbour's building.
Read the guideHow long does a two-storey extension take?
A two-storey extension usually takes about 9 to 12 months from first survey to completion, of which the build itself is typically 14 to 20 weeks, or 16 to 24 weeks for larger or more complex projects.
Read the guideShould I build a two-storey or a single-storey extension?
A two-storey extension usually gives you more space for the money than a single-storey one, because the foundations and roof serve two floors instead of one, but it costs more in total and is harder to get through planning.
Read the guideShould I build a two-storey extension or convert my loft?
A loft conversion suits you if your roof has enough head height and you want an extra bedroom without losing garden, while a two-storey extension suits you if you need new space on both floors, such as a larger kitchen and a bedroom.
Read the guideCan I build an extension over my garage?
Yes, you can usually build an extension over an attached garage, but a structural engineer first has to check that the garage foundations and walls can carry a new storey, and they often need strengthening.
Read the guideWhat design rules apply to a two-storey side extension?
A two-storey side extension is usually expected to look subordinate to the house, which in practice means setting it back from the front wall, setting its roof down below the main ridge and matching the roof shape and materials.
Read the guideDoes a two-storey extension add value to my home?
A two-storey extension usually adds value, because it adds floor area and often a bedroom or bathroom: Nationwide's research found 10% more floor area was associated with about 5% higher value, and adding a double bedroom and bathroom to a three-bedroom, one-bathroom house was associated with up to 24% higher value.
Read the guideCan I live at home during a two-storey extension?
Yes, most people live at home during a two-storey extension, because most of the work happens outside until the builder breaks through into the house near the end.
Read the guideWhat foundations and structure does a two-storey extension need?
A two-storey extension needs foundations designed for the weight of two storeys and a roof, usually concrete strip or trench-fill foundations, or a raft or piles where the ground is poor.
Read the guideReady to see what a second storey could do for your home?
Book a free site visit. We'll look at your house, talk through options and explain the approvals you'd need, then price the design stage in writing.