Unit B, Sarnia House, St Peter Port
Not marked as sole agency on the original listing, so this property may also be marketed by other agents not captured in this data.
What would improving it be worth?
Value added is the extra floor area priced at £6,093/m² — the midpoint asking price per m² among 58 St Peter Port 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 |
£85,300 | £70,000 14 m² · mid spec new build range £56,000–£84,000 |
+£15,300 | 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 |
£121,857 | £60,000 20 m² · mid spec refurb range £40,000–£80,000 |
+£61,857 | 97% (148) |
| Two-storey extension Roughly 14 m² on each of two floors. ~28 m² |
£170,600 | £140,000 28 m² · mid spec new build range £112,000–£168,000 |
+£30,600 | 93% (86) |
| Garage, annexe or outbuilding Detached space is worth less per m² than main-house floor area. ~18 m² |
£65,803 | £90,000 18 m² · mid spec new build range £72,000–£108,000 |
−£24,197 | 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.
Building work on this property
Architects with planning applications on file in the same parish as this property — not a claim any of them worked on this specific property, just a proximity signal:
Same estimate model this site uses on its planning pages (Channel Islands-adjusted UK cost ranges) — a rough starting point, not a quote. Always get quotes from a Guernsey builder before budgeting.
See also all Guernsey property professionals — architects, surveyors, mortgage brokers and trades.
Sale history
| Sale date | Price | At 2026 prices | Address on register |
|---|---|---|---|
| 2025-09-04 | £2,040,000 | £2,024,039 -1% | SARNIA HOUSE (PART BUILDING) GRAND BOUET / ELIZABETH AVE ST PP |
| 2025-09-02 | £2,040,000 | £2,024,039 -1% | SARNIA HOUSE ( part building) GRAND BOUET/ ELIZABETH AVENUE ST PP |
Matched by house name/street and parish against the States of Guernsey property register — not a unique-ID lookup, so a shared house name could span more than one property. The "2026 prices" column carries each price forward at the island-wide repeat-sales index — it is what that sum of money would be at today's market level, not a valuation of this property, and it takes no account of the condition it was in then or any work done since.
Planning history
| Application | Description | Status |
|---|---|---|
| FULL/2026/0696 | Install ramp and balustrade to north elevation. | Granted Full record Decision notice (PDF) |
| FULL/2025/1573 | Install replacement windows to all elevations of turret at third floor/roof level. (Protected Building). | Granted Full record Decision notice (PDF) |
| FULL/2025/0260 | Alteration to entrance to south (front) elevation and install door to north (rear) elevation. Worked on by: Lovell Ozanne and Partners Surveying Limited | Granted Full record Decision notice (PDF) |
Matched by house name/street and parish against Guernsey planning applications decided between 25 Aug 2021 and 2 Jul 2026 — coverage is partial and not a unique-ID lookup. No bare decision date is available in this data, so the application number (which embeds its year) is shown instead. Where a "Worked on by" name appears, it's a known architect or surveyor cross-referenced by the same application number — worth a call before commissioning a fresh survey.
Search full planning history for this address (live, back to 2009)
Estimated cost of live planning permissions
Guernsey planning permissions lapse 3 years after grant if unbuilt; these applications fall inside that window and were not refused, so they are presumptively still capable of being implemented.
- FULL/2026/0696 — Internal alteration / change of use
£18,750–£50,000Install ramp and balustrade to north elevation. - FULL/2025/1573 — Window / door replacement
£10,000–£25,000Install replacement windows to all elevations of turret at third floor/roof level. (Protected Building). - FULL/2025/0260 — Internal alteration / change of use
£18,750–£50,000Alteration to entrance to south (front) elevation and install door to north (rear) elevation.
The work type shown is inferred automatically from the wording of the planning application's own description — it isn't stated by the applicant. The base cost range comes from published UK national renovation/build-cost guides for that type of project, and a ~25% uplift has been added to reflect higher shipping/import and skilled-labour costs in the Channel Islands — this is an estimate, not a published index. These figures are indicative only, not a quote: actual cost depends on specification, finish, ground conditions and the contractor. For anything serious, get a quote from a local quantity surveyor or builder.
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