What is an extension, loft or annexe actually worth in Jersey?
Value added, likely build cost, and how often Jersey actually approves that kind of application — the three numbers that decide whether a project is worth starting. Adjust the assumptions and watch them move.
Modelling Green Road Corner, Greve D'azette, JE2, St. Clement — £1,000 pcm, 60.4 m², St Clement. The value-added column below uses that parish's own price per m².
What would improving it be worth?
Value added is the extra floor area priced at £7,609/m² — the midpoint asking price per m² among 10 St Clement properties currently for sale that publish a floor area. Cost uses £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 Jersey actually approves that kind of application.
Unlike the value side of this table (priced from Jersey's own current asking prices), the £4k–6k new-build / £2k–4k refurbishment rates are carried across from guernseyproperties.gg, where they were supplied by that site's owner from local Guernsey experience — they have not been independently verified for Jersey, and construction costs plausibly differ between the islands. Treat the Cost and Net columns as an unverified placeholder until a Jersey-specific quantity surveyor's rate replaces them.
| Improvement | Value it might add | Likely cost | Net | Approved |
|---|---|---|---|---|
| Add a bedroom (single-storey extension) A modest double plus circulation, built new. ~14 m² |
£106,522 | £70,000 14 m² · mid spec new build range £56,000–£84,000 |
+£36,522 | 86% (569) |
| 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 |
£152,174 | £60,000 20 m² · mid spec refurb range £40,000–£80,000 |
+£92,174 | 86% (84) |
| Two-storey extension Roughly 14 m² on each of two floors. ~28 m² |
£213,043 | £140,000 28 m² · mid spec new build range £112,000–£168,000 |
+£73,043 | 83% (193) |
| Garage, annexe or outbuilding Detached space is worth less per m² than main-house floor area. ~18 m² |
£82,174 | £90,000 18 m² · mid spec new build range £72,000–£108,000 |
−£7,826 | 82% (216) |
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 that state a floor area — a minority of Jersey's live stock, so treat the sample sizes shown as real limits, not decoration.
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 far more than it actually is — a bigger bed-count usually means 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 — the type-of-work approval rates show how often that kind of application actually succeeds.