Cost Comparison: Extension Types for North London Homes

Introduction

One of the most common questions at the start of an extension project is: which extension type offers the most floor area for the budget? The answer depends on the specific site constraints, planning context, and the intended use of the new space. This guide provides a direct comparison of the main extension types available to north London houses — cost per sqm, total cost for a typical area, planning risk, and value added.

Single-Storey Rear Extension

The most common extension type for Victorian and Edwardian terraces in north London. A single-storey rear extension extends the ground floor backward into the rear garden, typically creating an open-plan kitchen-dining-living space.

Parameter	Detail
Typical floor area added	20–50 sqm
Construction cost (mid spec)	£2,800–£3,500/sqm
Total cost (30 sqm, mid spec)	£84,000–£105,000
Planning risk	Low–Medium (PD possible in many cases)
Value added	High — addresses primary functional deficiency

Two-Storey Rear Extension

A two-storey extension adds floor area at both ground and first floor levels. Requires planning permission in most cases (exceeds single-storey PD limits). More cost-effective per sqm than a single-storey extension because the fixed costs are spread over more area.

Parameter	Detail
Typical floor area added	40–80 sqm (ground + first floor combined)
Construction cost (mid spec)	£2,400–£3,200/sqm
Total cost (50 sqm combined, mid spec)	£120,000–£160,000
Planning risk	Medium — requires planning permission, 45-degree rule applicable
Value added	High — adds significant floor area and bedroom/bathroom capacity

Side Return Extension

Many Victorian terraces have a narrow side return — the strip of ground beside the kitchen that was originally used for the WC outbuilding and coal storage. Filling the side return with a single-storey extension widens the ground floor by typically 2–3m.

Parameter	Detail
Typical floor area added	8–20 sqm (side return infill)
Construction cost (mid spec)	£3,000–£4,000/sqm (premium for narrow site complexity)
Total cost (12 sqm, mid spec)	£36,000–£48,000
Planning risk	Low–Medium (often PD on semi-detached)
Value added	Good — transforms kitchen space and light quality

Loft Conversion — Dormer

A dormer loft conversion is typically the most cost-effective way to add a bedroom and en-suite in a Victorian or Edwardian terrace house. Often achievable within permitted development limits on the rear slope.

Parameter	Detail
Typical floor area added	20–40 sqm (usable loft floor area)
Construction cost (mid spec)	£2,200–£3,000/sqm
Total cost (25 sqm usable, mid spec)	£55,000–£75,000
Planning risk	Low (PD on rear slope in most cases)
Value added	High — adds bedroom category step-change

Basement Extension

A basement extension creates substantial additional floor area below ground level. The highest-cost extension type per sqm but the only option for adding significant floor area without reducing garden or when above-ground extensions are exhausted.

Parameter	Detail
Typical floor area added	40–150 sqm (under garden, under house)
Construction cost (mid spec)	£4,500–£6,000/sqm
Total cost (60 sqm, mid spec)	£270,000–£360,000
Planning risk	High — Camden and Islington have restrictive basement policies
Value added	Variable — strong in high-value locations, lower ROI elsewhere

Outbuilding / Garden Studio

A separate outbuilding or garden studio at the bottom of the garden — as a home office, gym, or additional living space — is often achievable within Class E permitted development rights without planning permission.

Parameter	Detail
Typical floor area	10–30 sqm (Class E PD limit)
Construction cost (mid spec, timber frame)	£2,000–£3,000/sqm
Total cost (20 sqm, mid spec)	£40,000–£60,000
Planning risk	Low (Class E PD applies in most cases)
Value added	Moderate — valued by buyers, does not increase bedroom count

Comparison Summary

Extension Type	Cost Efficiency	Planning Risk	Value Added
Single-storey rear	Medium	Low–Medium	High
Two-storey rear	Good	Medium	High
Side return	Medium	Low–Medium	Good
Loft conversion (dormer)	Good	Low	High
Basement extension	Low (high cost/sqm)	High	Variable
Outbuilding	Good	Low	Moderate

Conclusion

The right extension type for a specific property depends on a combination of planning opportunities, site constraints, budget, and the homeowner's priorities. For most north London Victorian terraces, the single-storey rear extension and the dormer loft conversion provide the highest value per pound invested. Basements are the right choice where significant floor area is needed and above-ground options are exhausted, but should be approached with a clear understanding of the higher costs and planning risk involved. An architect assessing a property for the first time will identify which extension types are feasible within the planning constraints of the specific site and provide a realistic comparison of cost and value for each option. See our detailed guides on dormer loft conversions and basement extensions for full technical and cost detail on each project type.