How to Assess Whether a House Has Good Extension Potential (Before You Buy)
By Emily Barnes, ARB Architect & Founder of The Potential House
If you’re buying a house with hopes of extending it — whether a rear extension, a side return, a wrap-around or even a loft/roof extension — the stakes are high. The difference between a fantastic renovation and an expensive mistake almost always comes down to what’s realistically possible with that specific house, that plot, and that budget.
As an architect specialising in pre-purchase advice, I regularly walk into homes where buyers are imagining a bright new kitchen, a beautiful open-plan layout or a bigger family living space — only to discover constraints that will dramatically change the plan, the cost, or even the decision to buy.
On the flip side, I also walk into houses where buyers can’t see the potential at all — and an extension (or a more modest intervention) could transform the property into something exceptional.
This guide is everything I look at as an architect when assessing extension potential before buying — written clearly, honestly, and with the aim of helping you make an informed decision.
Why Extension Potential Matters Before You Buy
Extensions aren’t cheap — especially in 2025. They also aren’t always necessary, or even the best-value route to getting the home you want.
Understanding what’s possible before you commit to a property helps you:
- avoid buying a house with hidden, expensive constraints
- judge whether the potential gain is worth the cost
- compare two properties objectively
- negotiate the purchase price more confidently
- avoid “money pit” situations
- plan properly from day one
You can also explore similar topics here:
- Buying a House to Renovate in the UK – Checklist
- Should You Speak to an Architect Before You Buy a House?
- Pre-Purchase Architect Appraisal Guide
1. First Test: Can the Garden or Plot Actually Take an Extension?
A good extension should add space, value, and functionality — without making the plot feel cramped.
Good signs
- Decent garden length
- Garden proportionate to the house size
- Side access or a side alley
- No major overshadowing from neighbours
- Plot wide enough for a side return or wrap-around
- Neighbouring houses with similar extensions (precedent)
Warning signs
- Garden already very small
- Neighbours’ extensions boxing you in
- Boundary constraints limiting depth or height
- Irregular garden shapes
- Extensions that would remove usable outdoor space
2. Flow & Connectivity: Will the Extension Feel Part of the House?
An extension only works if it connects naturally to the existing rooms.
- Logical circulation — no awkward dog-legs or dark inner rooms
- Cohesive connection between old and new spaces
- Avoiding “bolt-on” rooms that feel disconnected
- Avoiding dark internal zones created by deep extensions
3. Light & Orientation: Will the Extension Steal or Add Daylight?
Good signs
- South or west-facing garden
- Light already reaching deep into the plan
- Potential for rooflights or internal glazing
- Scope for cross-views or dual-aspect spaces
Issues to watch
- North-facing gardens
- Deep extensions without overhead light
- Overshadowing from neighbouring houses
- Narrow Victorian or Edwardian plans
Permitted development depth guide:
- Terraced / Semi-detached: up to 6m
- Detached: up to 8m
Extensions at these depths require rooflights or internal glazing.
4. Access & Logistics: The Hidden Cost Multiplier
Access can affect cost as much as the design itself.
- Can machinery reach the garden?
- Will excavation spoil be carried through the house?
- Is there space for material storage?
- How easy is skip access?
Poor access = more labour + more disruption + higher costs.
5. Structure: What’s Supporting What?
One of the biggest misconceptions:
“We’ll just knock that wall down.”
The feasibility and cost depend on exactly what the wall is carrying.
- Walls supporting multiple floors require major intervention
- Joist direction determines ease of forming openings
- Previous DIY structural work often needs correcting
- Roof structure type (trussed vs cut roof) affects loft potential
6. Drainage & Services: A Quiet But Crucial Factor
If kitchens or bathrooms move, drainage becomes a major cost variable.
- Location of soil stacks
- Shared Victorian drains in side returns
- Falls and depths of existing runs
- Feasibility of keeping plumbing in similar zones
7. Cost vs Value: Will the Extension Pay You Back?
When I assess this, I consider:
- Ceiling value of the street
- Neighbouring precedents
- Your likely timeframe in the house
- Whether internal reconfiguration offers better value
- “Space over spec” — square metres add more value than finishes
8. What Extension Work Might Cost (My Early Heuristics)
- Single-storey rear extension: ~ £45–50k per 10m²
- Large 6–8m extension: requires rooflights and careful daylighting
- Side-return: £60–90k+ depending on structure/drainage
- Wrap-around: £90–150k+
- Double-storey: £70–80k outside London; £120–140k in London/SE
Additional cost drivers:
- Excavation
- Drainage relocation
- Difficult access
- Bespoke glazing
- Conservation area constraints
Professional fees: 10–15% of project
Contingency: ~20%
9. Common Traps Buyers Miss
- Losing too much light in the existing rooms
- Narrow, awkward spaces from poor side-return planning
- Major steel requirements
- Drainage conflicts
- Party Wall costs and delays
- Awkward junctions with neighbours’ extensions
- Compromised garden proportions
10. Architect’s Rules of Thumb (Quick Checks)
GOOD SIGNS ✓ Wide plot ✓ Garden large enough ✓ Side access ✓ Good roof height ✓ Similar extensions nearby ✓ South/west-facing garden WARNING SIGNS ✗ Small garden ✗ Overshadowing ✗ Neighbour's deep extensions ✗ Poor access ✗ Structural walls carrying multiple floors ✗ Victorian shared drains
11. Types of Extensions (Quick Overview)
- Rear extension: best for family living/kitchen space
- Side return: great for terraces, but daylight is critical
- Wrap-around: transformative but complex
- Double-storey: good value if you need multiple rooms
- Loft or roof extension: often best £/m² – check roof form
FAQ — Extensions & Pre-Purchase Advice
Do I need an architect before buying if I plan to extend?
If you’re considering major reconfiguration or extension, yes. An architect can assess feasibility, structure, planning risk and cost before you commit. See: Should You Speak to an Architect Before You Buy a House?
Is a pre-purchase appraisal worth it?
Yes — it can prevent you buying a house that can’t deliver what you’re hoping for. See: Pre-Purchase Architect Appraisal Guide
How do I know if a house is extendable?
Check precedent on the street, plot size, daylight constraints, and planning designations.
Will an extension always add value?
No — it depends on ceiling values, proportion, and whether the design solves real problems.
Should I consider alternatives before committing to an extension?
Often yes. Lofts and internal reconfiguration can offer better value. See: Buying a House to Renovate – Checklist
Book a Free 15-Minute Call
If you’re viewing a renovation property and want to understand its true potential before you commit, I offer a free, no-obligation pre-purchase call.
You can ask anything — layout potential, planning risk, structural issues, likely costs — and I’ll give you clear, honest, architectural guidance.