Double Storey Side Extension
Building up the side of a semi or detached house is often the neatest way to add a bedroom: a garage or side passage becomes a utility, study or downstairs shower room, with a bedroom and en suite above.
£1,700 to £2,900 per m²two storey extension, both floors counted, national 2026 range including VAT
It always needs planning permission in England
Permitted development does not allow a side extension of more than one storey, so a double storey side extension needs a householder application. The fee in England is £548. That is not a hurdle to fear; side extensions are among the most common applications councils see. It does mean the design should be shaped by what your council's guidance asks for from the start.
What councils usually look for
Most councils publish householder design guidance, and for two storey side extensions it tends to make the same points. Read your own council's version; these are the ones to expect:
- Setting the front of the upper floor back from the front wall of the house, so the extension reads as an addition.
- A ridge a little lower than the main roof, for the same reason.
- A gap between the upper floor and the side boundary, so a row of semis does not join up into a terrace.
- No side windows that overlook the neighbour, or obscure glazing where a window is needed.
- Materials, roof shape and window proportions that match the house.
What fits in the space
The width you have is the width between the house and the boundary, less the gap your council asks for upstairs. A 2.5 m wide side extension gives a single bedroom or a generous en suite above; at 3.5 m or more you get a double bedroom with a shower room. Downstairs, the same width makes a utility and WC, a study or a wider hall.
The new bedroom is reached from the existing landing, so check where a doorway can go through the side wall upstairs before settling the layout.
Side extensions on a drive suit a crane lift. Get a price for both floors.
Get a quoteBuilding close to the boundary
Side extensions are often built on or near the boundary line, which brings in the Party Wall etc. Act: notices for building on the line and for digging foundations close to your neighbour's. Gutters and roof edges must not overhang their land without agreement. Sharing one party wall surveyor with next door usually costs £700 to £1,500 for each neighbour.
Think about access for maintenance too. A wall built tight to the boundary can only be painted or repointed from next door.
Why the side often suits prefab
A side extension usually sits next to a drive, which is the best crane access a house can offer: the crane stands on the road or drive and sets panels or modules straight into place without lifting over the roof. Budget about £1,400 for a day's contract lift, meaning a 25 t crane and the crew who plan and sling each load. Watch for overhead cables, trees and parking restrictions on the street, and tell the firm about them when you ask for a quote.
What a double storey side extension costs
Expect £1,700 to £2,900 per m² for two storeys with both floors counted, as a national figure with VAT. A side extension 3 m wide and 6 m deep has 18 m² on each floor, 36 m² in all, which puts it roughly between £61,000 and £104,500. Converting or demolishing a garage, moving a gas meter or drains, and a new en suite all push towards the top of that range.
Use the cost calculator with your own width and depth.
Scotland and Wales
Rules differ. In Scotland, see Circular 1/2024 and allow time for a building warrant, which you need before building. In Wales, see the Welsh Government guidance. In both, check the current planning fee when you apply.
Sources
- GOV.UK permitted development technical guidance.
- Planning Portal, planning application fees, fee table from 1 April 2026.
- GOV.UK, Party Wall etc. Act 1996 explanatory booklet.
- Build costs: published 2026 UK extension cost guides, national, including VAT.
Questions families ask
Can I build a two storey side extension under permitted development?
No, not in England. Side extensions under permitted development are limited to a single storey, so a double storey side extension needs a householder application.
How close to the boundary can a two storey side extension go?
There is no single national figure; it comes from your council's design guidance and the planning decision. Many councils ask for a gap at first floor level. Building on or near the line also brings in party wall notices.
Can I build over my garage instead?
Often, if the garage walls and foundations can take the load, which an engineer must check. A first floor over a garage of about 25 m² typically costs £45,000 to £75,000, nationally, including VAT.
Read next
- Planning for two storeys
The rules for rear extensions and when a certificate helps.
- First floor extensions
A bedroom on top of a garage or an older single storey part.
- 2 storey extension cost
Price your own width and depth.
Get a quote for your two storey extension
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