Extension or Loft Conversion: Choosing the Right Investment for Thames Valley Homes
Extensions add horizontal space and usually cost more. Loft conversions add vertical space and often cost less per square metre. Your answer depends on how your family uses the house, your garden size, your roof pitch, your planning context, and your budget. This guide compares the two options across the factors that matter to Thames Valley homeowners.
Key takeaways
- •Single-storey rear extensions in the Thames Valley cost 2500 to 3500 GBP per sqm in April 2026. Loft conversions cost 1800 to 2600 GBP per sqm.
- •Loft conversions add the value of a bedroom and sometimes a bathroom. Extensions usually add a kitchen, living, or dining footprint.
- •Most single-storey rear extensions fall under permitted development. Most loft conversions need planning permission except for simple roof-light conversions.
- •Extension build time is typically 10 to 16 weeks. Loft conversion time is 8 to 12 weeks.
- •Garden size, roof pitch and head height, and existing staircase run are the three biggest physical constraints.
What each option actually gives you
A rear extension usually extends the kitchen and dining space or adds a family room. The result is more horizontal open-plan space on the ground floor. A loft conversion usually adds a bedroom, an en-suite, or a study in the roof space. The result is an extra floor. Which one helps depends on what your family is short of. If you need more bedrooms, a loft is the direct answer. If you need a bigger kitchen-dining area for how you actually live, an extension is the direct answer.
Cost comparison in the Thames Valley
Single-storey rear extensions in Slough, Maidenhead, Reading, and across the Thames Valley cost roughly 2500 GBP to 3500 GBP per square metre for standard finishes in April 2026. A 20 sqm rear extension lands at 50000 GBP to 70000 GBP. Loft conversions cost 1800 GBP to 2600 GBP per square metre. A 25 sqm dormer loft lands at 45000 GBP to 65000 GBP. Double-storey extensions sit at 2000 GBP to 2800 GBP per square metre because the roof is only built once across two floors. Prices depend heavily on finish, roof type, structural complexity, and site access.
Planning and permitted development
Most single-storey rear extensions on detached and semi-detached houses fall under permitted development rights, within size limits set by the General Permitted Development Order. You still need a Lawful Development Certificate to evidence compliance and building regulations approval for the build. Loft conversions are trickier. A simple roof-light conversion (no dormers, no change to roof shape) usually falls under permitted development. Any dormer, hip-to-gable, or mansard typically needs planning permission. Conservation areas and Article 4 Directions disable permitted development rights and force a full application.
Timeline and disruption
A single-storey rear extension takes 10 to 16 weeks on site. The kitchen is typically unusable for 3 to 6 of those weeks. A loft conversion takes 8 to 12 weeks on site and is less disruptive because the work happens above the main living space. The staircase install is the single most intrusive week in a loft conversion. Double-storey extensions run 14 to 20 weeks and disrupt both floors. New builds are a different category, typically 6 to 12 months depending on size.
Resale value. What Thames Valley buyers pay for
Loft conversions reliably add 15 percent to 20 percent to a 3-bed semi when they add a bedroom and en-suite. The uplift is strongest when the conversion takes a house from 3 to 4 bedrooms. Kitchen-dining extensions add 5 percent to 10 percent and are valued more for lifestyle than for extra bedroom count. In higher-value Thames Valley postcodes like SL4, SL6, SL7, SL9, and HP9, the loft-bedroom multiplier is larger because bedroom count crosses family-size thresholds. In rental markets like RG1, RG6, and SL1, bedroom count drives yield directly.
Physical constraints to check before you commit
For a loft conversion, measure internal ridge height. You need at least 2.2 metres from joist to ridge at the highest point. Check roof pitch: pitches below 30 degrees are hard to convert without a dormer. Check your existing first-floor staircase: the loft stair has to land somewhere that does not destroy the first floor plan. For an extension, measure usable garden depth, check boundary setbacks, and identify any drains, mains, or mature trees that constrain the footprint. Both paths need a structural engineer input early.
How Asuka247 helps you decide
We visit the house, measure the loft and garden, photograph the existing constraints, and walk you through both options with realistic cost ranges for your specific property. If one path is clearly wrong for your house, we tell you. We do not sell projects that will not deliver the value you want. Once you pick a direction, we coordinate architectural drawings, planning or permitted development, structural engineering, and the full build through to handover.
Frequently asked questions
Can I do both an extension and a loft conversion?
Yes, and many Thames Valley homeowners do. A rear extension plus loft conversion is a common 2-phase project. We usually recommend doing both in one programme because scaffolding, site access, skip logistics, and party wall agreements all overlap.
Do I need planning permission for a dormer loft conversion?
Usually yes. A rear-facing dormer might fall under permitted development within strict volume limits, but side-facing and front-facing dormers almost always need planning permission. Conservation areas always require full planning. Check your property boundaries and constraints before committing.
What is the highest-value loft conversion type?
A mansard with en-suite, done well, usually returns the most. It gives the largest usable floor area, full head height, and can accommodate a proper bedroom plus en-suite plus storage. It also needs planning permission in nearly every case.
How much deposit do you take?
Asuka247 runs a payment schedule tied to construction milestones. Typical structure: deposit on contract signing (10 to 15 percent), stage payments at groundworks, first fix, second fix, and practical completion. We never take more than 30 percent before work starts on site.
Do you work in conservation areas?
Yes. Extensions and loft conversions in conservation areas need careful material choice, massing, and consent coordination. We work across conservation areas in Windsor, Eton, Marlow, Beaconsfield, Old Amersham, Henley-on-Thames, and the various Thames-side settings.
Which Thames Valley areas do you build in?
Slough, Maidenhead, Windsor, Reading, Bracknell, Wokingham, Ascot, Newbury, High Wycombe, Marlow, Beaconsfield, Amersham, Henley-on-Thames, Staines, Egham, plus Eton, Burnham, Taplow, Iver, and Stoke Poges.
Need help on your block?
Asuka247 delivers Fire Safety Act 2021 remediation across the Thames Valley. Call 07931 118 247 or book a site survey.
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Published 2026-04-17. Last updated 2026-04-17.