What we know, what we estimate, and what we don’t.

How an assessment is produced

1. You upload a photograph of your house. A vision model reads visible characteristics — house form, wall material, roof form, windows, any existing extension — and locates where an extension would sit in the photo.
2. An image-editing model repaints only that region to show a concept extension. The rest of the photo is masked so your house is preserved.
3. You describe the house and what you want from it: type, rough measurements, objectives, budget, and a planning checklist where “I don’t know” is always allowed.
4. A deterministic model — ordinary, inspectable rules, not AI — selects extension types that suit your house and priorities, sizes them, prices them, screens them against the planning conditions, and compares them. The same inputs always give the same output.

The numbers in your assessment come from step 4, not from an AI model. That is deliberate: costs and planning conditions should be reproducible and explainable, not generated.

Provided, observed, assumed, calculated, unverified

Every material input in your assessment carries one of five labels, shown in a table at the top of the assessment and in the report:

• Provided — you told us (a measurement, the house type, a planning-checklist answer).
• Observed — suggested by your photo. A photo cannot establish measurements, foundations, drains, boundaries or legal status.
• Assumed — a typical value used because the fact is unknown, such as a 7.5m house width. It always says so.
• Calculated — produced by a fixed formula from the above: footprints, floor areas and cost ranges.
• Unverified — needs a survey, a professional or the local authority: the planning route, boundary positions, drainage and structure.

The property overview also separates three kinds of statement:

• Observed — read from your photo or stated by you. “Walls appear to be painted render.” “You estimated the garden at 14m deep.”
• Inferred — a reasonable conclusion from what was observed. “As a terraced house, any extension is likely to sit against a party wall.”
• Unknown — what nobody has measured or checked. “Actual site dimensions have not been surveyed.”

Where something is unknown, we lower confidence rather than substitute a guess. Where a typical value has to be used to size a concept (a house width, for example), the assessment says so.

Dimensions

Concept dimensions are derived from the measurements you provide and from typical proportions for each extension type. A single-storey rear extension, for example, is sized where possible to stay within the 40m² exempted-development limit and to leave at least 25m² of rear garden. We do not measure anything from your photograph — a photo cannot give reliable dimensions. All dimensions are indicative concept dimensions to be verified on site.

Cost methodology

Construction is priced per square metre of gross floor area using ranges commonly cited for Irish domestic extensions in 2025–26. They are not derived from tender prices for your site and we do not claim a published source for any single figure. Base construction rates (including VAT at 13.5%, before adjustment):

Adjustments: Dublin +10%; areas outside the main cities −7%; standard specification −8%, high specification +15%; projects under 15m² +15% because fixed costs don’t shrink with floor area.

On top of construction, the total project budget adds:

• Design and planning drawings: 7–12% of construction, including VAT at 23%.
• Structural engineer, measured survey, and planning costs (statutory fee and notices; or confirmation of exempt status).
• Building Control (BCAR) assigned certifier where the extension is over 40m².
• Allowances for feature glazing, external works and making good, and — where your objectives include them — a kitchen and a bathroom.
• Contingency of 10 / 12 / 15% of construction, fees and allowances — not of the kitchen and bathroom fit-out, which are supply-and-fit prices.

Kitchen and bathroom allowances are only added to options that actually build a kitchen or a shower room — a dormer bedroom is never charged for a kitchen. Two-storey options are priced on the floor area of both floors at a lower base rate, which reflects the shared foundations and roof. VAT is included once: 13.5% within construction rates, 23% within professional fees.

Three scenarios are shown: lower, typical and higher complexity. Each line is rounded (to €1,000, or €100 for small items) and the total is the sum of the rounded lines, so the breakdown always adds up to the figure shown. Excluded: furniture and appliances outside the kitchen allowance, works to the rest of the house, abnormal ground conditions, rerouting a public sewer, and inflation. Only priced drawings and competitive tenders produce a real cost.

Planning screening

Each option is screened against the conditions for extensions to the rear of a house in Class 1, Part 1, Schedule 2 of the Planning and Development Regulations 2001 (as amended), as we understand them in October 2026. Where we have the information, we test: total floor area including earlier extensions (40m² — see below on the reported 2026 change); upper-floor area (12m² for terraced and semi-detached houses, 20m² for detached); a 2m separation from boundaries for upper floors; and at least 25m² of private open space remaining to the rear. We treat side extensions, the side element of wraparounds and dormers as generally outside these exemptions. A protected structure, an Architectural Conservation Area or a condition removing exempted-development rights all point to a planning application.

The result is one of three routes — potentially exempt, subject to verification; planning application likely; or cannot determine from the available information — with a confidence level, a condition-by-condition table (appears met / not met / unknown) and a list of issues to verify. Two or more unknown material conditions mean we say we cannot determine the route. We never give high confidence to an exemption, because the regulations contain conditions — window separation distances, wall and roof heights — that cannot be checked from a photograph. Planning exemption is not Building Regulations compliance: structure, fire safety, drainage and energy rules apply either way, and extensions over 40m² fall under Building Control (BCAR).

Unconfirmed 2026 change. Irish planning publications report that S.I. 338 of 2026 raised the rear-extension limit from 40m² to 45m² from 27 July 2026 and added classes for dormers and rooflights. We have not been able to verify the official text, so an extension between 40m² and 45m² is screened as “cannot determine”, and dormers are still treated as likely to need permission. Ask your architect or council which limit applies. Read the planning guide.

How options are ranked

Objective fit ×2, minus garden-impact and complexity penalties, plus or minus planning route, minus a penalty where the budget is exceeded. Options within 0.3 points are treated as tied.

In full: score = 2 × objective fit (0–6), minus a garden-impact penalty (0 / 0.4 / 0.8, or 0 / 1 / 2 when keeping the garden is a priority), minus a complexity penalty (0 / 0.4 / 0.8), plus 0.6 if potentially exempt or minus 0.3 if an application is likely, minus 1.5 if the typical budget exceeds 115% of your stated budget (or 0.8 if even the lower budget exceeds it). The highest score leads. When another option is within 0.3 points, we say the options are effectively tied and set out the trade-off instead of manufacturing a winner. The matrix, the option cards and the recommendation all read from the same scores.

Sources and review dates

How we use AI

• Image generation: an image-editing model (FLUX Fill Pro, run via Replicate) repaints a masked region of your photo to show a concept. Images can contain errors and are illustrative only.
• Photo reading: a vision language model describes the visible house and suggests where an extension would sit. Its observations are labelled as such and can be wrong.
• Not AI: option selection, dimensions, costs, planning screening, comparison and recommendation are rule-based and reproducible.

Your photos and data

• Your photo is stored so we can render concepts and show them back to you. Generated images are stored at unguessable addresses.
• There are no accounts: your project is reachable by anyone who has its link, which contains a random 122-bit identifier that cannot be guessed or listed. Treat it like a password and share it only with people you trust. Project pages are never indexed by search engines.
• You can delete your photo and every generated image at any time from your project page. A minimal record of any payment is kept for accounting.
• Analytics are cookieless and never include your photo, its address, your email or anything you type.
• Full details: privacy policy.

Refunds

If a package does not deliver what it describes, reply to your receipt email and we will fix it or refund you in full. Upgrades are charged the difference against what you have already paid. A refunded order removes the features it paid for.

Professional limitations

• Visualisations are conceptual images generated from your photo. They are not architectural drawings.
• Dimensions are indicative and have not been surveyed. Verify on site.
• Cost estimates are indicative ranges, not quotations, and carry no guarantee.
• Planning screening is not planning or legal advice and does not guarantee that any route applies or that permission will be granted.
• No structural assessment has been carried out.
• No measured survey has been carried out.
• Local and site-specific conditions can materially change the outcome.
• Professional advice is required before design is finalised and before any construction.

ExtensionVision is preparation for professional advice, not a substitute for it. Before you commit, engage a registered architect or architectural technologist, and a structural engineer where structure is affected. We have no commercial relationship with any professional or builder.