Budget
£22,500
- Focused essentials
- Practical finishes
Loft Conversion vs Extension Costs in Northern Ireland
Estimates derived from UK trade benchmark data and regional labour indices, updated May 2026. Methodology →
Loft Conversion vs Extension in Northern Ireland often runs below mainland UK averages for similar specifications. Our UK guide still frames the work; this page is how we express that market on the ground.
In Northern Ireland, local rates are often below UK mainland averages for similar work. For the full UK-wide baseline, compare with Loft Conversion vs Extension Cost UK.
Two ways to take action on Loft Conversion vs Extension costs
Pick the path that fits where you are — running early numbers, or pressure-testing a quote you've already got.
Typical Northern Ireland loft conversion vs extension budgets
Three planning tiers for loft conversion vs extension in Northern Ireland, with scope and a representative figure for each. Run your own numbers in the calculator for a tailored range.
Mid-rangeMost common
£39,500
- Balanced specification with core upgrades
- Reliable materials
Premium
£58,000
- Premium materials
- Wider scope with higher coordination demands
Typical regional cost ranges
| Item | Cost Range |
|---|---|
| Loft conversion (Velux / rooflight) | £14,100 – £28,000 |
| Loft conversion (dormer) | £23,500 – £47,000 |
| Loft conversion (mansard) | £37,500 – £66,000 |
| Single-storey extension (20 m²) | £26,500 – £51,500 |
| Single-storey extension (30 m²) | £37,500 – £70,500 |
Mini Loft Conversion vs Extension cost calculator
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What's included in Northern Ireland loft conversion vs extension costs
- Loft — no loss of garden; often permitted development; roof structure must suit; disruption mainly inside.
- Extension — uses garden; may need planning; foundations and build cost; more ground-floor space.
- Value add — loft often adds bedroom + en-suite; extension adds kitchen-diner or living; both can add 10–20% to value.
- Cost per m² — loft often £1,000–£2,500/m²; extension £1,200–£2,800/m²; loft can be better value where viable.
- Timeline — loft typically 4–10 weeks on site; extension 8–16 weeks.
- When to choose loft — need a bedroom, roof suits, want to keep garden. When to choose extension — need ground-floor space, kitchen-diner, or no loft potential.
5 line items every fair Loft Conversion vs Extension quote should include
Use this checklist to spot missing scope before you sign — each item names what should be priced and what to ask for if it isn't.
- 1
Cost comparison — loft vs extension per m²
Loft conversion: £1,800-£2,500/m² (dormer); £1,200-£1,800/m² (Velux only); £2,500-£3,500/m² (mansard). Extension: £2,400-£3,500/m² (single-storey rear); £2,800-£4,000/m² (two-storey side); £4,000-£6,000/m² (London). Loft typically 25-35% cheaper per m² for similar usable area.
Fair UK range: Loft typically £1,800-£2,500/m² vs Extension £2,400-£3,500/m² for same usable area.
Ask: What's the per-m² rate for each option, and how does that compare to my £/m² benchmark?
- 2
Garden loss impact — extension only
A 4m × 5m extension (20m² added) consumes 20m² of garden. In a small garden, that's 30-50% of usable outside space. Loft conversions don't touch the garden. For families who use the garden, this is a major decision factor — and adds 'invisible' £-cost via reduced enjoyment and resale impact.
Fair UK range: Garden loss can reduce property value by 5-15% in family areas where outdoor space is at premium.
Ask: What's the garden currently used for, and how much will I lose to an extension? Have I asked an estate agent about local garden value?
- 3
Planning permission likelihood
Loft conversions: usually permitted development (no planning needed) if within 40m³ (terraces) or 50m³ (semi/detached) and not on principal elevation. Extensions: rear single-storey up to 4m (detached) or 3m (semi/terrace) is permitted; everything else needs planning (£462, 8 weeks). Conservation areas restrict both.
Fair UK range: Loft: 80% permitted development. Extension: 50% permitted development depending on size and area.
Ask: Does each option fall within Permitted Development limits, or does either need full planning?
- 4
Build duration + disruption
Loft conversion: 8-12 weeks typical. Disruption mainly to top floor and access. Can usually live in property throughout. Extension: 12-20 weeks typical. Disruption to ground floor (especially if knocking through to existing kitchen). Often need to move out for 4-8 weeks of structural phase.
Fair UK range: Loft 8-12 weeks; Extension 12-20 weeks. Extension may require £4-£12k in temporary accommodation.
Ask: What's the realistic build duration for each option, and can I live in the property throughout?
- 5
Resale value uplift comparison
Loft conversion adding bedroom + ensuite: typical £40-£60k value uplift on £40-£60k spend (1:1 ROI, breakeven). Extension adding kitchen-diner: typical £30-£60k uplift on £50-£80k spend (0.6-0.75:1 ROI). BUT in London/SE, both can hit 1.5-2x ROI. Get local estate agent valuations BEFORE committing.
Fair UK range: Loft: £40-£60k value uplift typical (national); Extension: £30-£60k uplift typical (national). London 50-100% higher.
Ask: What's the local value uplift for each option? Get 2-3 estate agent valuations before committing.
Want this run on your actual Loft Conversion vs Extension quote? Upload it and our AI Quote Checker flags missing line items, overcharges and the questions worth asking.
7 red flags that mean you might be overcharged on a Loft Conversion vs Extension quote
UK-specific signals — each red flag explains why it matters and the question that surfaces the truth.
Contractor steers toward their specialism without explaining trade-offs
Why it matters: Loft specialists will steer toward loft; extension contractors will steer toward extension. Reputable advisors lay out both options with honest cost/value/disruption comparison and let you decide. Single-option-only quotes mean you're not getting independent advice.
Ask: Can you give me both a loft option AND an extension option for comparable extra space, with cost + duration + value comparison?
No mention of head-height check for loft viability
Why it matters: Loft conversions need 1.5m+ head height across 50% of the floor area for habitable use. Many UK lofts (especially Victorian terraces with shallow pitched roofs) don't qualify. A loft 'specialist' who doesn't check this first is unprofessional.
Ask: What's the existing head height in the loft? Loft conversion needs 1.5m+ across 50% of floor for habitable use.
Extension proposal without considering loft alternative
Why it matters: If both options are viable, the extension at 25-35% higher cost should be justified by clear advantages (specific use case for ground-floor space, family layout reasons). A contractor who doesn't even discuss the loft alternative is biased.
Ask: Have you considered whether the same usable space could come from the loft instead, at lower cost?
No estate agent valuation built into the decision
Why it matters: Local market matters. In some areas, a loft bedroom adds more value than a ground-floor extension; in others, the opposite. Without 2-3 local estate agent valuations of post-build property under each option, the decision is uninformed.
Ask: Have I got estate agent valuations for the property post-loft AND post-extension? The market verdict matters.
Loft quote significantly below £1,500/m² for dormer
Why it matters: UK 2026 typical for dormer loft conversion is £1,800-£2,500/m². Below £1,500/m² usually means corner-cutting on insulation, fire compliance, structural engineering, or Building Regs.
Ask: How are you achieving this loft price? What's included for structural engineer, Part B fire compliance, Part L insulation?
Extension quote significantly below £2,000/m² (outside London)
Why it matters: UK 2026 typical for single-storey extension is £2,400-£3,500/m² (more in London). Below £2,000/m² usually means: missing professional fees, Party Wall Awards, structural engineer, Part L insulation upgrade, or quality finishes.
Ask: How are you achieving this extension price? What's included for structural fees, Party Wall, and finishes?
No discussion of disruption or temporary accommodation
Why it matters: Extensions typically require 4-8 weeks of moving out during structural phase (knock-through to existing kitchen). At £800-£2,500/month, that's £4-£12k of additional cost. Lofts rarely require moving out. A contractor who doesn't discuss this hides a real cost.
Ask: Do I need to move out during the extension, and what's the cost of temporary accommodation?
Spot a couple of these on your Loft Conversion vs Extension quote? Upload it for a full red-flag scan and fair-rate comparison.
How to negotiate a Loft Conversion vs Extension quote
A simple framework, a verbatim script you can paste into an email or text, and the topic-specific levers that move the price.
Framework
- 1Get THREE quotes from contractors who can do BOTH loft and extension (or one specialist of each). Specify identical scope: same usable m² added, same use type, same finish level. Without comparable scope, the per-m² comparison is misleading.
- 2Demand both options costed in writing for each contractor: 'What would loft option X cost? What would extension option Y cost?' Reputable contractors do both honestly; biased ones avoid the unfavoured option.
- 3Get 2-3 local estate agent valuations of the property under each post-build scenario. The value-uplift difference often makes the decision clearer than the cost difference.
- 4Compare ALL costs not just build: include temporary accommodation (extension only), garden value loss (extension only), planning fees if needed, Party Wall fees if attached. The total comparison can shift 20-30% from headline build cost.
Verbatim script
I'm comparing a loft conversion to a rear extension to give us an extra bedroom and bathroom. Could you quote for the loft option (assume dormer with ensuite, ~25m² usable), and tell me what scope I'd need to ask an extension contractor for to be comparable? I want a like-for-like comparison: build cost, professional fees, contingency, total time, disruption (need I move out?), and your honest view on which adds more local value.
Topic-specific levers
- Cost-driven decision: loft dormer is typically 25-35% cheaper per m² than equivalent extension. If budget is tight and head-height permits, loft wins.
- Garden-driven decision: in family areas where outdoor space is at premium, extension often hurts resale value. Loft preserves garden — wins for value.
- Use-driven decision: extra bedroom + ensuite (sleeping space) loft is ideal — top of house, separated. Extension better for kitchen-diner / family room (ground floor flow).
- Planning-driven decision: if conservation area or Article 4 area restricts loft, extension may be the only option (and vice versa). Check planning constraints first.
- ROI-driven decision: get 2-3 estate agents to value the property under each scenario. In London/SE, both options typically positive ROI; in other areas, one is often clearly better.
Want to know which line items on your Loft Conversion vs Extension quote are above market before you negotiate? Upload it for a fair-rate comparison.
10 questions to ask before hiring a loft conversion specialist or extension contractor
Vet on competence, insurance, paperwork and process — not price alone. Each question spells out the answer you want and why.
1. Have you completed BOTH loft conversions AND extensions of similar scope in the last 18 months?
Why it matters: Specialists in only one option will bias the recommendation. Contractors who do both honestly can give comparative advice.
2. Can you provide cost + value estimates for BOTH loft and extension options with this property?
Why it matters: Reputable contractors give both options. Single-option quotes mean you're not getting independent comparison.
3. Have you done a head-height survey of the loft?
Why it matters: 1.5m+ head height across 50% of floor is mandatory for habitable loft. A contractor who skips this and proposes loft conversion is wasting your time.
4. Are you a member of the FMB (Federation of Master Builders), TrustMark, or relevant trade body?
Why it matters: Both loft and extension are high-value projects (£40k-£100k+). FMB/TrustMark membership signals competence and IBG warranty access.
5. Who's your structural engineer for each option, and is each IStructE-registered?
Why it matters: Both lofts and extensions need structural calculations. Engineer should be IStructE-registered for both options.
6. What contract are you proposing — JCT Minor Works for both options?
Why it matters: JCT contracts protect on £30k+ projects. Both loft and extension exceed that threshold.
7. What's the realistic disruption for each option — can I live in the property?
Why it matters: Lofts usually allow living-in; extensions often require moving out for 4-8 weeks. Cost difference matters.
8. Have you got local estate agent contacts who can value the post-build property under each scenario?
Why it matters: Local market verdict is the deciding factor for ROI. Reputable contractors have agent relationships; cowboys don't.
9. What's your warranty, and is it insurance-backed for both option types?
Why it matters: Industry norm: 10-year IBG for structural; 12-24 months for other workmanship. IBG matters because contractors fail.
10. Are you VAT registered, and what's your public liability cover (£5M+ for £50k+ projects)?
Why it matters: VAT for invoicing/warranty enforcement. PL ≥£5M industry norm for £50k+ projects.
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