Rear extension in St Helens: Planning Permission Guide
Everything you need to know about building a rear extension in St Helens, including permitted development rules, typical costs, and which professionals you'll need.
St Helens has conservation areas including St Helens Town Centre, Billinge, Rainford, and Newton-le-Willows. Listed buildings cluster around St Helens Parish Church and around the glass-making heritage. Substantial Green Belt covers the rural fringes. The borough is predominantly former mining and industrial terrace housing with later suburban development. Flood zones apply along the Sankey Brook valley.
Personalised odds
5 nearest comparables & refusal reasons for your address.
Get my report — £9See a sample →Recent rear extension decisions in St Helens
The 5 most recent decided applications. Addresses redacted to postcode area.
Single storey rear extension following removal of existing conservatory along with part garage conversion.
Erection of a part two storey part single storey rear extension, and demolition of the existing porch to the side replacing the existing …
Single storey rear infill extension
Single storey rear extension following the demolition of existing single storey rear extension and conservatory
Single storey side and rear extension.
These are borough-wide. See the 5 nearest to your address.
Get my report — £9Permitted Development Rules
Enlargement, improvement or other alteration of a dwellinghouse
GPDO 2015, Schedule 2, Part 1, Class A
Rear extensions fall under Part 1, Class A of the General Permitted Development Order (GPDO). This means you can often build one without a full planning application, provided your project stays within the dimension limits.
Key dimension limits
Single storey
Two storey
These are the maximum GPDO limits for a detached house with no constraints. Your actual limits depend on your property type, location, and any planning restrictions.
Enter your postcode to see the personalised limits for your rear extension.
Check my limitsTypical Costs in North West England
Construction costs for rear extensions in North West England vary depending on size, specification, and whether you're building single or double storey.
Cost per m²
construction rate
Typical total
Based on 12–18 m²
Cost per m²
construction rate
Typical total
Based on 20–35 m²
Typically includes
Typically excludes
Double storey extensions have a lower per-m² cost as foundations and roof are shared across two floors.
Get a personalised cost estimate based on your actual rear extension dimensions.
Get my estimateWhich Professionals Do You Need?
The professionals you'll need depend on the size and complexity of your extension, your property type, and whether you share walls with neighbours.
You'll need a party wall surveyor for this project.
A draughtsperson or architectural technician can prepare the plans needed for a single storey extension. A full architect may not be necessary.
Typical fee: £1,500–£3,500 (Plans + Building Regs)
Single storey extensions typically need structural calculations for any openings in existing walls and for foundations.
Typical fee: £500–£1,500
Your semi-detached house shares a boundary with neighbours. Extensions and loft work within 3 metres of a shared wall trigger the Party Wall Act.
Typical fee: £700–£1,500 (per neighbour)
This guidance is based on a typical semi-detached house with no special constraints. Listed buildings, conservation areas, and attached properties may require additional professional input.
Find out exactly which professionals you need for your rear extension.
Get personalised guidanceConservation Areas & Listed Buildings
If your property is in a conservation area, the maximum depth for a single storey rear extension is reduced to 3 metres for all property types. Two storey rear extensions are not permitted in conservation areas.
Properties in conservation areas often have reduced permitted development rights. Some project types (like side extensions) lose PD rights entirely. An Article 4 Direction can remove additional PD rights.
Listed buildings (Grade I, II*, or II) have no permitted development rights at all. Any external alteration requires Listed Building Consent in addition to any planning permission. An architect experienced in heritage work is essential.
Check if your property is in a conservation area or has other planning constraints.
Check my propertyPrior Approval (Larger Home Extension Scheme)
Single storey rear extensions between 4m and 8m deep (detached) or 3m and 6m deep (other houses) may be built under the "larger home extension" scheme. This requires a Prior Approval application with a neighbour consultation — not a full planning application. The fee is lower and the decision is faster.
Frequently Asked Questions
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