How to Do a Property Development Appraisal Before You Buy

A development appraisal is a simple financial model with three components:

**1. Gross Development Value (GDV)**: The total value of the completed, fully developed property. This is the output — what you will sell or refinance against at the end of the project.

• For a conversion or extension project:
• •GDV = sale value of the finished property (or the remortgage value if you are refinancing to retain)
• •Use current comparable sales (Rightmove, Zoopla, Land Registry) for completed, equivalent properties in the area — same size, same finish level, similar location within the area
• •Be conservative — use the midpoint of the comparable evidence, not the aspirational top of market

**2. Total Development Cost (TDC)**: Every cost of the project. For an existing property purchase and conversion/extension:

• *Acquisition costs*:
• •Purchase price
• •SDLT (Stamp Duty Land Tax) — at current rates; add 3% SDLT surcharge for additional properties/investment purchases
• •Legal fees (typically £1,500–£3,000 for residential purchase)
• •Surveyor fees (Level 2 or Level 3 RICS survey, typically £600–£1,500)
• •Mortgage arrangement fee (if purchase is mortgaged)

• *Construction costs*:
• •Contractor's price (the main contract sum)
• •Contingency — typically 10–15% of the construction cost for unexpected items
• •Client-supplied items (kitchens, bathrooms, flooring, fittings) — often underestimated

• *Professional fees*:
• •Architect: typically 6–12% of construction cost for full service (planning + construction drawings + contract administration)
• •Structural engineer: typically 1–3% of construction cost or fixed fee
• •Party wall surveyor fees (if applicable): £1,200–£3,000
• •Planning application fees: £206 for householder application (2025 England); higher for other application types
• •Building Control fees: typically 1–2% of construction cost (private approved inspector) or £400–£1,200 for local authority

• *Finance costs*:
• •If the project is mortgage-financed: arrangement fee + interest during the build period
• •Bridging finance (for development/conversion projects): arrangement fee 1–2%; monthly interest 0.5–1.2%; exit fee 0–1%
• •For cash buyers: opportunity cost of capital deployed (typically modelled at 5–7% p.a. for a UK cash investor in 2025)

• *Disposal costs (if selling)*:
• •Estate agent commission: 1–3% of sale price
• •Legal fees for sale: £1,000–£2,500
• •Marketing and staging costs: typically £1,000–£5,000

**3. Profit margin**: Profit = GDV − TDC

• For residential development, a common target is:
• •Return on cost (profit/TDC): 20–25% for a viable development
• •Return on GDV (profit/GDV): 15–20%

If the appraisal shows less than 15% on GDV, the margin is thin — and any cost increase or delay will erode it significantly.

**Scenario**: Purchase a 2-bedroom Victorian terrace in South East London, add a rear extension (extending the kitchen and adding a third bedroom), and sell.

• **GDV**:
• •3-bedroom terrace, fully refurbished with extension, in this location: £650,000
• •Conservative estimate after reviewing 10 recent comparable sales in the area

**TDC**:

• *Acquisition costs*:
• •Purchase price: £520,000
• •SDLT (3% surcharge, investment property): £27,600 (on £520k at 2025 SDLT rates including surcharge)
• •Legal fees: £2,500
• •RICS Level 3 survey: £1,000
• •Mortgage arrangement fee: £2,000
• **Subtotal acquisition: £553,100**

• *Construction costs*:
• •Rear extension (5m × 4m, single storey, full refurbishment of existing interior): £145,000 (contractor quote for structural extension + kitchen + bathrooms + redecorating throughout)
• •Client-supplied kitchen and bathrooms: £18,000 (allowance)
• •Contingency (10%): £14,500
• **Subtotal construction: £177,500**

• *Professional fees*:
• •Architect (full service, 8% of construction): £11,600
• •Structural engineer: £2,500
• •Building Control (private inspector): £1,200
• •Planning application fee: £206
• •Party wall surveyors: £2,400
• **Subtotal professional: £17,906**

• *Finance costs* (12-month project, bridging at 0.8%/month on 70% LTV):
• •Loan drawn: £364,000 (70% of £520k purchase price)
• •Monthly interest: £2,912
• •12-month interest: £34,944
• •Arrangement fee (1.5%): £5,460
• •Exit fee (1%): £3,640
• **Subtotal finance: £44,044**

• *Disposal costs*:
• •Estate agent (1.5%): £9,750
• •Legal fees for sale: £2,000
• **Subtotal disposal: £11,750**

**Total Development Cost: £804,300**

• **Profit:**
• •GDV: £650,000
• •TDC: £804,300
• •**Profit: (£154,300) — NEGATIVE**

• **This project does not work**. The purchase price is too high relative to the potential GDV. To make the project viable at this specification and GDV:
• •The purchase price needs to fall by approximately £120,000–£150,000 (to approximately £370,000–£400,000); or
• •The GDV needs to be significantly higher (£800,000+ — which would require a more ambitious extension or a higher-value comparable set); or
• •The construction cost needs to be significantly lower — or a cheaper specification

This is the most important function of a development appraisal: it tells you what to pay for the property, not what you feel it is worth.

A development appraisal is only as good as the assumptions in it. The assumptions that matter most — and that are most commonly wrong — are:

**Construction cost**: The most frequently underestimated item in amateur appraisals. Homeowners and inexperienced developers routinely use headline cost-per-m² figures (e.g., '£2,000/m² for an extension') without understanding what is and isn't included. Professional fees, client-supplied items, contingency, and service upgrading are commonly excluded from headline rates.

**How to stress test**: Run the appraisal at construction cost +20%. If the project still works, it is robustly viable. If it fails at +20%, the margin is too thin.

**GDV**: The most commonly optimistic assumption. Developers often use the top comparable, or a comparable from a slightly better location, and ignore that their specific property and location may command a lower value.

**How to stress test**: Run at GDV −10%. If the project fails at GDV −10%, the upside is too dependent on the market staying favourable.

**Finance cost**: Interest-only finance for 12 months at 0.8%/month on a £364,000 loan costs £35,000. Many amateur development appraisals omit this or significantly underestimate it. A 6-month delay to the programme (common on projects with planning delays, scope changes, or contractor issues) adds £18,000 to the finance cost — which can eliminate the profit on a thin-margin project.

**SDLT surcharge for investment purchases**: First-time buyers and single-home-owner purchasers pay lower SDLT. Anyone purchasing an additional residential property pays a 3% SDLT surcharge on the full purchase price. This is commonly forgotten in back-of-envelope appraisals and can add £10,000–£30,000 to acquisition cost on a London purchase.

**The 'right price' principle**: The development appraisal tells you the maximum you should pay for the property, given a realistic GDV, realistic costs, and your target profit margin. If the seller's asking price exceeds this maximum purchase price, the project doesn't work at that price — and no amount of optimism about GDV or cost reduction will fix a fundamentally wrong entry price.