Loft Conversion vs Rear Extension London (2026): Costs, Planning & Which to Choose
Two of the most popular ways to add space in London. Here's a complete comparison of costs, planning routes, permitted development eligibility, and value added.
TL;DR - The Quick Answer
A loft conversion adds a bedroom or bathroom for £54,000–£75,000 and usually falls under permitted development (up to 40-50m³ volume). A single-storey rear extension costs £40,000–£65,000 and creates open-plan living space - also often PD-eligible (up to 3-4m deep depending on property type). A two-storey rear extension (£80,000–£130,000) nearly always needs planning permission but maximises floor area gain. Your choice depends primarily on: what type of space you need, your budget, and whether your property already has loft space.
Loft Conversion vs Rear Extension: At a Glance
| Loft Conversion | Single-Storey Rear | Two-Storey Rear | |
|---|---|---|---|
| Typical cost | £54,000–£75,000 | £40,000–£65,000 | £80,000–£130,000 |
| Planning route | Usually PD | Usually PD | Almost always planning |
| PD eligible? | Yes (up to 40–50m³) | Yes (up to 3–4m deep) | Rarely |
| Adds bedrooms? | Yes (1–2) | No (living space) | Yes (with planning) |
| Disruption level | Medium | Medium | High |
| Build time | 8–12 weeks | 8–10 weeks | 16–24 weeks |
| Value added | £40k–£80k | £25k–£50k | £60k–£100k |
Source: Mayfair Studio project data • Data as of 2026
Most London homeowners in 2-bed terraces opt for a loft conversion to create a third bedroom - this is typically the highest-value move per £ spent. Families needing open-plan kitchen-diner space lean toward a rear extension. Both are valid; your property type and neighbourhood often dictate which is more feasible.
Costs in Detail
Extension and loft costs in London vary by property type, specification, and borough. Here are the typical ranges for 2026, inclusive of VAT:
| Extension Type | Cost Range | Notes |
|---|---|---|
| Loft conversion (hip-to-gable) | £60,000–£85,000 | Requires full planning permission |
| Loft conversion (dormer) | £54,000–£72,000 | Often permitted development |
| Loft conversion (Velux/roof light) | £28,000–£45,000 | Usually PD, minimal disruption |
| Single-storey rear extension | £40,000–£65,000 | PD route common |
| Two-storey rear extension | £80,000–£130,000 | Almost always needs planning |
Add architect fees of £1,295–£4,000 (Mayfair Studio from £1,295, including planning and structural drawings). Structural engineer: £1,500–£2,500. Planning fee if required: £258 (householder) or £578 (full planning, April 2026 rates).
Planning Permission and Permitted Development
Planning rules for loft conversions differ from extensions:
Loft conversions
- –Hip-to-gable and rear dormers: Usually permitted development under Class B, subject to volume limits
- –Volume limit: 40m³ for terraced houses, 50m³ for detached and semi-detached
- –Mansard conversions: Nearly always require planning permission
- –Conservation areas and Article 4 zones: PD rights removed - planning required
- –Listed buildings: Listed building consent always required
Rear extensions
- –Single-storey: PD up to 4m deep (detached) or 3m deep (terraced/semi) under Class A
- –Two-storey: Rarely PD - almost always requires full planning permission
- –Conservation areas: Most extensions require planning permission
- –Article 4 directions: Remove PD rights in many London boroughs
Mayfair Studio's AI checks your property's planning constraints instantly - free, no contact details needed.
Which Adds More Value: Loft or Extension?
In London, adding a bedroom typically adds more value per £ spent than adding floor area alone. A bedroom makes your property appeal to a larger pool of buyers. That said, it depends on your current configuration:
- –2-bed property → loft conversion creates 3rd bedroom: usually highest ROI, can add £50k–£80k value
- –3-bed property with small kitchen → rear extension to open-plan kitchen-diner: strong ROI, adds £30k–£60k value
- –3-bed property in high-value area → two-storey rear adds bedroom + living space: highest total value, £60k–£100k uplift
Mayfair Studio from £1,295 - includes planning drawings, structural calculations, and building control.
Which Should You Choose?
Use this decision guide:
| Your Situation | Recommended Option |
|---|---|
| You need a bedroom | Loft conversion |
| You need open-plan living space | Single-storey rear extension |
| You want maximum floor area | Two-storey rear extension |
| You want minimum planning hassle | Loft conversion (PD route) |
| You're in a conservation area | Get advice first (both can be restricted) |
| Budget under £55,000 | Single-storey rear or Velux loft |
| Budget over £80,000 | Two-storey rear or dormer loft |
A loft conversion costs £54,000–£75,000 and reliably adds a bedroom - usually under permitted development. A rear extension costs £40,000–£130,000 depending on storeys. In London, adding a bedroom typically gives the best return per £ spent. Mayfair Studio handles both from £1,295.
Frequently Asked Questions
Should I get a loft conversion or rear extension in London?
For most London homeowners, a loft conversion is the better investment - it adds a bedroom for £54,000–£75,000 and usually qualifies as permitted development (no planning needed). A rear extension is better if you need open-plan living space rather than a bedroom. Single-storey rear extensions cost £40,000–£65,000; two-storey cost £80,000–£130,000 but almost always require planning permission. Mayfair Studio handles both types from £1,295.
Is a loft conversion cheaper than a rear extension in London?
A loft conversion (£54,000–£75,000) is typically more expensive than a single-storey rear extension (£40,000–£65,000), but cheaper than a two-storey rear extension (£80,000–£130,000). Velux loft conversions start from £28,000. For equivalent floor area added, loft conversions cost more per m² but create more valuable bedroom space.
Which adds more value - loft conversion or extension in London?
A loft conversion usually adds more value per £ spent in London because it creates an additional bedroom. A dormer loft conversion adds £40,000–£80,000 to property value. A single-storey rear extension adds £25,000–£50,000. A two-storey rear adds £60,000–£100,000. For 2-bed properties, a loft conversion creating a 3rd bedroom typically gives the highest ROI.
Do I need planning permission for a loft conversion in London?
Most loft conversions in London do not need planning permission if they fall within permitted development limits: maximum 40m³ volume increase for terraced houses, 50m³ for detached and semi-detached. However, conservation areas, Article 4 direction zones, and mansard conversions always require planning permission. Approximately 60% of London loft conversions proceed under permitted development.
How long does a loft conversion take vs a rear extension?
A loft conversion typically takes 8–12 weeks to build. A single-storey rear extension takes 8–10 weeks. A two-storey rear extension takes 16–24 weeks. If planning permission is required (rather than PD), add 8–12 weeks for the planning process before construction starts.