New Build vs Extension: How to Decide What's Right for Your Property

**Why extension is often financially superior to moving in London**:

Moving house in London is one of the most expensive transactions a homeowner can make. The total transaction cost of selling one house and buying another includes:

• •Stamp Duty Land Tax on the new purchase (2% on the tranche £250,001–£925,000; 5% on £925,001–£1,500,000; 12% above £1,500,000)
• •Estate agent fees on the sale: 1–2% of sale price + VAT
• •Legal fees: £1,500–£5,000 (solicitor for sale and purchase)
• •Survey on new property: £400–£1,500 (RICS Level 2 or Level 3)
• •Removal costs: £800–£3,000
• •Mortgage arrangement fees, early repayment charges on existing mortgage: variable
• •Premium for the same size house in a slightly better location (the 'why move' cost): typically significant

• *Total transaction cost*: For a London homeowner selling a £700,000 house and purchasing a £900,000 house (£200,000 uplift for the additional space):
• •SDLT on £900,000 purchase: £31,250
• •Agent fee on sale (1.5%): £10,500
• •Legal: £3,000
• •Survey: £1,000
• •Removals: £1,500
• •*Total transaction cost*: approximately £47,250 — before the £200,000 uplift itself

By contrast, a double-storey rear extension adding 40–50m² of additional space (a ground floor kitchen-dining room and a first floor bedroom) might cost £70,000–£100,000 in construction — adding the same or more space for significantly less total cost than the move.

**The extending calculation**:

• For many London homeowners, the decision matrix is:
• •Total cost to extend: construction + professional fees + temporary accommodation (if needed) + furniture and finishes
• •Total cost to move: transaction costs (above) + premium for the additional space in the new property + moving disruption

• If the extension cost is materially less than the move cost to achieve the same space — which it frequently is in London — extending is the financially superior route, provided:
• •The extended property would still be in the right location (the homeowners want to stay in the same street/school catchment)
• •Planning permission for the desired extension is obtainable
• •The existing house can physically accommodate the extension within the site constraints
• •The extended house value would not exceed the 'ceiling' for the street (the maximum price achieved by any comparable property on the same street or in the immediate area)

**The 'ceiling' constraint**:

The primary risk in extending is over-improving — spending money on an extension that does not add equivalent value because the extended property's value would exceed the local price ceiling. In a street where houses sell for £500,000–£600,000, spending £120,000 on an extension to create a house worth (in theory) £720,000 will not achieve that value — it will sell for approximately £600,000 (the ceiling) regardless of quality. The extension adds liveability and enjoyment, but the financial return is limited.

*Practical rule*: Before committing to a large extension, commission a desktop valuation from a local estate agent on the extended property — what would the house sell for after extension? If the estimated extended value minus the current value is less than the total extension cost, the extension destroys value financially (though it may still be the right personal decision).

**Garden sub-division and new build on plot**:

In areas of London where garden sizes are generous (1930s semis with long gardens; Victorian houses with large rear plots; any property in an outer London suburb with sufficient curtilage), it may be possible to sub-divide the garden and build a new dwelling on the land separated from the existing house.

This is fundamentally a development proposition rather than a home improvement: the homeowner is not improving their existing home, they are developing land to create a new separately-titled residential property that can be sold, let, or used by a family member.

• **When plot sub-division is viable**:
• •The existing garden is large enough to accommodate a new dwelling with its own private amenity space (typically a minimum of 50–100m² of private garden for a new 2-bed house in London, depending on the LPA)
• •The new dwelling can be accessed independently from the highway (its own front door/access; not routed through the existing house)
• •The new dwelling would not materially overlook or overshadow the existing house or neighbouring properties
• •The LPA's Local Plan does not prohibit back-garden development (some London Boroughs have explicit policies protecting back-garden land from development)
• •The new dwelling is of sufficient size and quality to add significant capital value

**The economics of plot sub-division**:

• For a 1930s semi in outer London with a 120m garden (measured from the rear wall of the house), sub-dividing the rear 60m to create a building plot and constructing a 2-bed house:
• •Value of 2-bed new-build house in outer London suburb: £350,000–£500,000
• •Construction cost (2-bed house, 70–80m²): £200,000–£280,000
• •Planning application and professional fees: £15,000–£30,000
• •Finance costs (if borrowing): variable
• •Effect on existing house value: building in the garden reduces the garden size and may negatively affect the existing house value by £20,000–£50,000 depending on the extent of the remaining garden

*Gross development value (GDV) of the new build minus construction and professional costs minus loss of existing house value = development profit*: In the best cases, this is positive and significant; in marginal cases (small plots, lower-value areas), the economics do not work.

**Planning for garden sub-division in London**:

• Planning permission is required. Key planning considerations:
• •Back-garden development policies: many London Boroughs have policies resisting the development of 'back-land' (gardens not facing the highway) on the grounds of urban character and amenity
• •Design quality: the new dwelling must be of high design quality, appropriate in scale and character to the surrounding area
• •Highways access: a new vehicle access (dropped kerb, new access route) requires highways approval from the London Borough; TfL approval is required for properties on the TLRN (Transport for London Road Network)
• •Biodiversity Net Gain: new development in England must achieve at least 10% Biodiversity Net Gain (mandatory from February 2024); a garden sub-division on a previously-green site typically has a BNG deficit that must be offset

**Demolish and rebuild**:

Demolishing the existing house and building a new, larger, more efficient dwelling on the same site is rarely financially viable in London — the demolition cost (£15,000–£30,000 for a standard terrace), planning requirement for the new dwelling, construction cost, and the loss of the existing house's contribution to the scheme rarely produce a better outcome than extending.

The exception is a property in a very poor structural condition (requiring more money to repair than it is worth) in a high-value area — where a quality new-build dwelling on the same site would achieve significantly higher value than the existing repaired property. This is rare but does occur.

**Beyond the financial analysis — why extension often wins**:

The financial case is important, but the decision to extend vs move is also deeply personal:

• •*Location continuity*: families who have established school places, social networks, and community connections in a specific street or area often place enormous value on not having to move — an extension at higher construction cost than the moving transaction cost may still be the right decision because of location continuity
• •*No moving disruption*: moving house is ranked as one of the most stressful life events; an extension — while disruptive during construction — avoids the transaction stress, school-change risk, and complete life disruption of a move
• •*Customisation*: an extension is designed and built around the specific needs of the family, whereas a move involves compromising with an existing property that may not be exactly right
• •*Known neighbours*: particularly relevant in London, where neighbour relationships significantly affect quality of life; homeowners with established good neighbour relationships are often reluctant to start afresh in a new street

**What to consider before extending**:

• •*Is the extension structurally and planning-viable?* Get a desktop planning opinion and a structural assessment before committing to design fees
• •*Is the construction budget realistic?* Use this guide and the cost guides at /cost-guides to form a realistic view of total cost before appointing an architect
• •*What is the construction programme?* Will you need to vacate the property? (Single-storey rear extensions to occupied houses can usually be built while the family remains in residence, with temporary kitchen facilities; loft conversions usually allow the family to remain; major full house refurbishments with M&E upgrade usually require temporary relocation)
• •*What is the ceiling for the street?* Understand the local market ceiling before designing an extension — design to a specification appropriate to the ceiling, not to aspirations that cannot be recovered in resale
• •*Is the designer and contractor team well-qualified?* A poorly managed extension causes more disruption and costs more than a well-managed one — invest in quality professional relationships from the start