Planning Rules
Semi-Detached House Permitted Development: Extensions, Lofts & More (2025)
Semi-detached houses have more PD flexibility than terraced houses — including the right to build a single-storey side extension — but the rules are strict to protect neighbours and the street scene.
⏱ 6 min read📅 Updated June 2025🏠 England only
Quick Answer
More flexibility than terraced — but side extensions must be single-storey
Semi-detached houses have the full range of permitted development rights, including the right to add a single-storey side extension under Part 1 Class A. The key restrictions are: single-storey side extensions cannot exceed half the width of the original house; two-storey side extensions always require planning permission; rear single-storey extensions up to 3m (or 6m under Prior Approval); and loft conversions up to 50m³. The shared party wall with the neighbouring semi means any extension involving the party wall will need to comply with the Party Wall etc. Act 1996.
Side Extensions
Unlike terraced houses, semi-detached houses have a PD right to build a single-storey side extension. The key conditions:
- Single-storey only: Two-storey or first-floor side extensions always require planning permission
- Width: No wider than half the width of the original house
- Height: No higher than 4m (single-storey)
- Position: Must not extend forward of the principal elevation (front of the house)
⚠️ The “half width” rule catches people out If your original house is 6m wide, a PD side extension can be no more than 3m wide. Many homeowners want the full side passage width, which can be 3–4m — check the original width carefully. The measurement is from the outside of the external wall.
Rear Extensions
| Extension type |
Semi-detached limit |
| Single-storey rear |
3m from original rear wall (6m under Prior Approval scheme) |
| Two-storey rear |
3m from original rear wall, 7m minimum from rear boundary |
| Wraparound |
Not permitted development — requires planning permission |
Loft conversions in semi-detached houses are permitted development under Part 1 Class B:
- Maximum addition of 50m³ of roof space (same as detached)
- No extension beyond the plane of the existing roof slope that fronts a highway
- Any roof extension must be set back 20cm from the eaves
- Side-facing windows must be obscure-glazed and non-opening below 1.7m
Rear dormers are the standard approach for semi-detached loft conversions — adding a large rear dormer to maximise headroom while keeping the front roof slope intact.
Party Wall Issues
Any extension or loft conversion that involves work to the shared party wall with your neighbour will be subject to the Party Wall etc. Act 1996. This requires you to serve a Party Wall Notice on your neighbour before starting work. If your neighbour consents, work can proceed after 14 days. If they dissent or don’t respond, a Party Wall Agreement (an award made by party wall surveyors) is required before work can start. Party wall issues are separate from planning — you can have PD rights and still need to comply with the Act.
Outbuildings and Garages
Outbuildings for semi-detached houses follow the same Part 1 Class E rules as other houses. The 50% curtilage coverage limit and height limits apply. A detached garage in the side of the plot is very common for semi-detached houses — check that the garage won’t project in front of the principal elevation, which would remove the PD right and require planning permission.
Frequently Asked Questions
Can I build a side extension on a semi-detached house without planning permission?
Yes — a single-storey side extension is permitted development under Part 1 Class A, provided it is no wider than half the original house width, no higher than 4m, and doesn’t extend in front of the principal elevation (front of the house). Two-storey side extensions always require planning permission. Check your planning conditions (for new builds) and whether you’re in a conservation area before relying on PD rights.
Can I extend my semi-detached house to the full width of the side passage?
Only if the side passage is no wider than half the original house width. If your house is 6m wide, a PD side extension can be maximum 3m wide. If the side passage is 3.5m wide and the house is 6m wide, the extension would need to be 3m wide or less to stay within PD limits — you’d have a 0.5m gap remaining. If you want to fill the full side passage width and it exceeds half the house width, you need planning permission.
Do I need the neighbour’s permission to extend a semi-detached house?
Not for planning purposes — planning permission (or PD) is a matter between you and the council, not your neighbour. However, if the extension involves work to or near the shared party wall, you must serve a Party Wall Notice under the Party Wall etc. Act 1996. Your neighbour can’t prevent you from building under PD, but they can trigger the party wall surveyor process, which must be resolved before work starts.
How much can I extend a semi-detached house under permitted development?
For a single-storey rear extension, up to 3m deep (or 6m under the Prior Approval neighbour consultation scheme). For a two-storey rear extension, up to 3m deep. For a side extension, up to half the original house width. For a loft conversion, up to 50m³ of additional roof space. These are the maximum PD limits — you may be able to go larger with a planning application. Always check for planning conditions and conservation area restrictions first.
More on Permitted Development Rights
Extensions, loft conversions, outbuildings, solar panels — our complete guide covers everything you can build without planning permission.
Read the Complete PD Guide →
