Should You Extend Your London Home or Sell and Move? A Financial Framework

Most homeowners significantly underestimate the transaction cost of moving in London. The total cost of selling and buying includes:

• **Stamp Duty Land Tax (SDLT)**: for a main residence:
• •Up to £250,000: 0%
• •£250,001–£925,000: 5%
• •£925,001–£1,500,000: 10%
• •Above £1,500,000: 12%

• Examples (purchasing property only — not your sale proceeds, which carry no SDLT for a main residence):
• •Buying at £600,000: SDLT = £17,500
• •Buying at £800,000: SDLT = £32,500
• •Buying at £1,000,000: SDLT = £43,750

**Estate agent fees (selling)**: typically 1.0–1.5% + VAT of the sale price. On a £700,000 sale: £8,400–£12,600.

**Conveyancing (both sale and purchase)**: combined typically £3,000–£6,000 including search fees, Land Registry, and legal fees.

**Mortgage arrangement fees**: if remortgaging to a larger mortgage, arrangement fees (typically £500–£1,500) plus any early repayment charges on the existing mortgage.

**Survey fees (buying)**: homebuyer survey or full structural survey on the new property: £500–£1,500.

**Removal costs**: £1,500–£5,000+ depending on distance and volume.

**Immediate works on the new property**: even a well-maintained second-hand property typically requires some improvement works — redecoration, new flooring, kitchen or bathroom updates: £10,000–£50,000+ is common.

**Total transaction cost of moving in London (typical range)**: £40,000–£100,000+

This is money that leaves the household permanently — it produces no asset, no improvement, and no return.

Extension costs are more visible because they are itemised in a contract. But they often feel larger than moving costs because they are not embedded in the price of an asset.

• Typical extension costs (London 2025):
• •Single-storey rear extension (3m x 5m): £45,000–£65,000
• •Side return extension: £50,000–£80,000
• •Double-storey rear extension: £100,000–£165,000
• •Loft conversion (dormer): £45,000–£90,000+

The key difference from moving costs: **the extension produces an asset**. The money you spend on an extension typically adds value to the property. In many London locations, the added value approaches or exceeds the construction cost — making the extension a near-neutral or positive-return investment, unlike the transaction costs of moving, which produce nothing.

• **Additional pre-construction costs** (not always presented in the initial estimate):
• •Architectural drawings and planning: £3,000–£8,000
• •Structural engineer: £1,000–£4,000
• •Building Control fees: £1,000–£3,000
• •Party wall surveyor (if required): £1,000–£3,000

These are real costs that add to the build figure but are still asset-producing.

**Scenario**: a family in a 3-bed London terrace currently worth £700,000 needs a 4th bedroom.

**Option A: Move to a 4-bed** Typical 4-bed London terrace cost: £1,000,000–£1,200,000 Sale of current property: £700,000 (net of agent fees ~£686,000) Additional spend required: £314,000–£514,000 Stamp duty on purchase at £1,000,000: £43,750 Agent fees on sale (£700k at 1.25% + VAT): ~£10,500 Conveyancing: £5,000 Survey + other costs: £3,000 Removals: £3,000 Immediate works on new property: £20,000 **Total transaction cost (money permanently lost): ~£85,000**

**Option B: Loft conversion (dormer) — add 4th bedroom with en-suite** Typical dormer loft conversion: £55,000–£80,000 Pre-construction (architect, structural, Building Control, party wall): ~£12,000 Total spend: **£67,000–£92,000** Value uplift from loft conversion: £50,000–£100,000 Net cost (spend minus uplift): £0–£42,000 (and the household stays in place)

• **The comparison**:
• •Moving costs £85,000 just in transaction costs — before the price uplift of the bigger house
• •The loft conversion costs £67,000–£92,000 total, adds value to the property, and delivers the required bedroom in-situ
• •In many cases the loft conversion is financially superior to moving — even before accounting for the stamp duty saving

**The exception**: if the fundamental problem is the size of the plot, the neighbourhood, or the street rather than just the number of rooms, moving is the right decision. The financial analysis favours extending when the constraint is rooms/space, and the homeowner wants to stay in the same location.

Extending is the stronger financial decision when:

• •The location is right — you want to stay in the same street, school catchment, or area
• •The property has extension potential (permitted development headroom, loft with adequate head height, garden space)
• •The additional cost of the larger property exceeds the extension cost significantly
• •The transaction costs of moving are prohibitive (especially in higher-value markets where SDLT is significant)
• •You have a mortgage at a competitive rate that you would lose on a move
• •The extension design delivers everything you need — bedroom count, open-plan space, or study

• Extending is less compelling when:
• •The issue is not rooms but the fundamental property — a narrow plot, poor garden, or structural limitations
• •The new location would change the property's value trajectory significantly
• •The extension costs approach or exceed the stamp duty saving — this tends to happen at lower price points where moving costs are smaller
• •The property has very limited extension potential due to planning constraints