17 Castle Street, Douglas
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 £3,229/m² — the midpoint asking price per m² among 6 Douglas properties currently for sale that publish a floor area. Cost uses real Isle of Man 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 Isle of Man 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 |
£45,207 | £52,500 14 m² · mid spec new build range £42,000–£63,000 |
−£7,293 | 0% (314) |
| 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 |
£64,582 | £41,000 20 m² · mid spec refurb range £33,000–£50,000 |
+£23,582 | 0% (46) |
| Two-storey extension Roughly 14 m² on each of two floors. ~28 m² |
£90,415 | £105,000 28 m² · mid spec new build range £84,000–£126,000 |
−£14,585 | 0% (66) |
| Garage, annexe or outbuilding Detached space is worth less per m² than main-house floor area. ~18 m² |
£34,874 | £67,500 18 m² · mid spec new build range £54,000–£81,000 |
−£32,626 | 0% (199) |
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 the Isle of Man'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 — what work of this kind actually gets approved here shows how often applications like yours succeed, and on what grounds the refusals go.
Building work on this property
Same estimate model this site uses on its planning pages (UK cost ranges with the Isle of Man's 18% uplift) — a rough starting point, not a quote. Always get quotes from an Isle of Man builder before budgeting.
See also all Isle of Man property professionals — architects, surveyors, mortgage brokers and trades.
Sale history
No prior sale found in the Isle of Man property register for this address.
Planning history
| Application | Description | Status |
|---|---|---|
| 22/01312/C | Change of use of the ground floor and first floor rear annex from Class 1.1 to Class 1.3 | Permitted Look up 22/01312/C on the official register |
| 22/00316/B | Alterations including replacement roof tiles, staircase, creation of window for existing doorway and removal of flue to ground and first flo… | Permitted Look up 22/00316/B on the official register |
Matched by house name/street and parish against Isle of Man planning applications decided between 4 Jan 2011 and 29 Dec 2023 (the government published no 2017 files, so that year is absent) — 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 the official register for anything since 2023
Estimated cost of live planning permissions
Isle of Man 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.
- 22/01312/C — Internal alteration / change of use
£17,700–£47,200Change of use of the ground floor and first floor rear annex from Class 1.1 to Class 1.3 - 22/00316/B — Single-storey extension
£53,100–£88,500Alterations including replacement roof tiles, staircase, creation of window for existing doorway and removal of flue to …
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, with an 18% uplift for the Isle of Man. That uplift is not a guess: AtkinsRealis's July 2026 interim report for the Isle of Man Treasury puts Manx construction costs 18% above the UK average — joint 16th most expensive place to build on the BCIS rankings, comparable with Outer London — driven by imported materials with no local competition and a shortage of skilled trades. Figures exclude VAT at 20% and professional fees, typically 12–18%. 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.
