What is an extension, loft or annexe actually worth in Guernsey?
Value added, likely build cost, and how often Guernsey grants that kind of application — the three numbers that decide whether a project is worth starting. Adjust the assumptions and watch them move.
What would improving it be worth?
Value added is the extra floor area priced at £6,271/m² — the midpoint asking price per m² among 120 Guernsey properties currently for sale that publish a floor area. Cost uses real Guernsey build rates: £4,000–£6,000/m² new build and £2,000–£4,000/m² refurbishment, depending on quality. Net is against the mid rate. The last column is how often Guernsey actually grants that kind of application.
| Improvement | Value it might add | Likely cost | Net | Granted |
|---|---|---|---|---|
| Add a bedroom (single-storey extension) A modest double plus circulation, built new. ~14 m² |
£87,790 | £70,000 14 m² · mid spec new build range £56,000–£84,000 |
+£17,790 | 97% (1,510) |
| Loft conversion with a bedroom Works inside an existing roof, so it prices as refurbishment rather than new build — which is why it usually wins. ~20 m² |
£125,414 | £60,000 20 m² · mid spec refurb range £40,000–£80,000 |
+£65,414 | 97% (148) |
| Two-storey extension Roughly 14 m² on each of two floors. ~28 m² |
£175,580 | £140,000 28 m² · mid spec new build range £112,000–£168,000 |
+£35,580 | 93% (86) |
| Garage, annexe or outbuilding Detached space is worth less per m² than main-house floor area. ~18 m² |
£67,724 | £90,000 18 m² · mid spec new build range £72,000–£108,000 |
−£22,276 | 90% (738) |
Two things it cannot see. First, it prices floor area at the local average rate — it doesn't know your plot, your roof, your outlook, or whether an extension is physically possible; nor whether your street has a ceiling price that no amount of extending beats. Second, the rate comes from asking prices of other properties currently for sale, because Guernsey's sold register records no floor areas at all — so it reflects what sellers ask, not what buyers paid.
The obvious alternative method is worse, which is why it isn't used here: comparing a typical 3-bed with a typical 4-bed suggests a bedroom is worth around £430,000 island-wide. It isn't — a 4-bed is generally a bigger house on a bigger plot, so that figure measures the mix of what's for sale, not the value of adding a room.
Before spending anything, get a builder's quote and talk to an architect — who is actually filing applications shows which firms are active and how often their applications succeed.