If you need more space in your home, two of the most popular routes are a loft conversion or a house extension. Both can add significant space and value, but they suit different circumstances, budgets, and goals. This guide gives you an honest comparison so you can make the right decision for your home.

The Key Differences at a Glance

Factor	Loft Conversion	House Extension
Direction of growth	Upward (using existing roof space)	Outward (into garden or side)
Planning permission	Often not needed (PD)	Often not needed (PD) for small extensions
Typical cost	£35,000 – £70,000	£35,000 – £90,000+
Space gained	Bedroom + bathroom (typically)	Kitchen, living, dining room (typically)
Garden impact	None	Reduces garden area
Disruption	Moderate (roof work, staircase)	Moderate to high (demolition, groundworks)
Value added	15–25%	8–20%
Best for	Adding bedrooms and bathrooms	Enlarging kitchen, living, dining spaces

What Does a Loft Conversion Add?

A loft conversion makes use of the existing roof space above your top floor. Depending on the type:

• Velux (rooflight) conversion: Adds bedrooms within the existing roof profile — cheapest option, limited headroom
• Dormer conversion: Adds a box-shaped dormer to the rear roof slope — full-height room, typically 1–2 bedrooms with en suite
• Hip-to-gable conversion: Extends the roof on a semi-detached or detached — usually combined with a dormer
• Mansard conversion: Rebuilds the rear roof to near-vertical — premium London option, maximum space

A loft conversion almost always adds bedroom and bathroom space, not living space. If what you need is a larger kitchen or an open-plan family room, a loft conversion is the wrong solution.

What Does a House Extension Add?

A house extension projects outward — typically to the rear or side. It works at ground level, which means it typically creates:

• Larger kitchen-diner
• Open-plan family living space
• Additional reception room or playroom
• Garden room or utility space
• Ground-floor bedroom or study

Double-storey extensions add both ground-floor living space AND an upper-floor bedroom or bathroom.

Cost Comparison

Loft Conversion Costs (2025)

• Velux/rooflight conversion: £20,000 – £35,000
• Rear dormer with bedroom and en suite: £35,000 – £60,000
• Hip-to-gable + rear dormer: £45,000 – £75,000
• Mansard conversion (London): £55,000 – £90,000+

House Extension Costs (2025)

• Single-storey rear extension: £35,000 – £65,000
• Single-storey with glazed roof and bi-folds: £50,000 – £85,000
• Side return extension (London): £35,000 – £70,000
• Double-storey extension: £75,000 – £130,000

At comparable quality, the costs are broadly similar — both options fall in the £35,000–£70,000 range for a well-executed single project. Larger or more complex versions of either type cost more.

Planning Permission

Both loft conversions and extensions often fall within permitted development rights for houses in England — meaning no planning application is required:

• Most rear dormers on terraced and semi-detached houses are permitted development (volume limits apply)
• Single-storey rear extensions up to 4m (detached) or 3m (other) are permitted development

Both require planning permission when: the property is listed, in a conservation area, or the works exceed PD limits. Front dormers always need permission; large double-storey extensions may need permission.

Disruption During Construction

Both are disruptive, but in different ways:

• Loft conversion: Roof opened up (risk of weather ingress); new staircase requires removing part of the landing or a room; existing bathroom may be affected
• Extension: Back of the house demolished; foundations dug; living in the house is possible but the kitchen may be unusable for weeks; major dust and noise

Many families temporarily relocate for a week or two during the most disruptive phases of either project. Extensions that involve kitchen removal typically require more temporary relocation than loft conversions.

Impact on Garden

A loft conversion has zero impact on your garden — the work is entirely in the roof. An extension reduces your garden footprint. If outdoor space is precious or your garden is already small, a loft conversion preserves it entirely.

Which Adds More Value?

Loft conversions (adding a bedroom + bathroom) tend to add a higher percentage return than equivalent-cost extensions:

• Rear dormer loft conversion: 15–25% value uplift
• Single-storey rear extension: 8–15% value uplift

This is partly because adding an extra bedroom to a house — say, converting a 3-bed to a 4-bed — steps the property into a higher buyer bracket. A larger kitchen-diner improves buyer appeal but doesn’t necessarily cross a bedroom-count threshold.

Can You Do Both?

Yes — and many homeowners do a phased programme of both. A common approach:

• Phase 1: Rear extension — create the open-plan kitchen-diner and connection to the garden
• Phase 2: Loft conversion — add the extra bedroom and bathroom

The two projects are independent structurally and can be done by different contractors at different times. Doing both in one build programme is possible and saves some preliminaries costs, but is significantly more disruptive.

Frequently Asked Questions

Is a loft conversion cheaper than an extension?

Not significantly — both typically cost £35,000–£70,000 for a well-executed single-room-and-bathroom project. Rooflight-only conversions are cheaper (£20,000–£35,000) but add limited usable space.

Which is less disruptive: loft conversion or extension?

Loft conversions tend to be slightly less disruptive to daily life — the main disruption is at the top of the house and the staircase installation, rather than demolishing the back of the house and kitchen. However, both require a few weeks of significant disruption.

I need a bigger kitchen, should I do a loft conversion or extension?

A rear extension. Loft conversions add space in the roof — typically bedrooms and bathrooms. If you need more kitchen or living space, an extension that enlarges the ground floor is the solution.

Which is better for a terraced house in London?

Both are excellent options for London terraces. Many London homeowners do both over time: a rear or side return extension first for the open-plan kitchen, then a loft conversion for the bedroom. For maximum return per pound, a loft conversion on a terraced house has historically delivered the best returns.

Can Crown Architecture help me decide which is best for my property?

Yes — we offer free initial consultations to discuss your space needs, budget, and what’s achievable for your property. Call Crown Architecture on 07443 804841.