We use HM Land Registry Price Paid data, so these are real sold prices rather than asking prices. Where there are enough recent sales of your property type nearby, we filter to those. If there aren't, we widen out to the local area and tell you we've done it.

Development type changes the figure. Conversions tend to sell slightly under comparable existing stock, because buyers know what the property used to be. New build usually sells above it, because people pay for new and for the warranty that comes with it. A refurb done properly should land around the local median.

Every appraisal gets scored out of 100 across six areas: profitability, planning risk, flood and environmental risk, construction risk, market demand and exit strategy. Profitability carries the most weight because it's the question that matters most.

A good headline score can still be hiding one weak category, which is why we show you all six rather than just the number.

These start from BCIS benchmarks, the same data quantity surveyors and lenders use when they're sense-checking a development finance application. BCIS prices a main contractor appointment — margin and preliminaries included — so we apply a 20% reduction to the national baseline before anything else. That reflects a self-managed development using your own subcontractors rather than a contracted main contractor, which is how most of our users actually build. If you're pricing on a full contract basis, expect to pay more than the figures we show.

Where you build makes a real difference. Labour, access and getting materials to site all cost more in London and the South East than they do in the North or the Midlands, so we adjust for region on top of that, rather than applying one national rate.

Already got a quote or your own costing? You can enter your own build cost instead of our estimate — we'll still show the benchmark range alongside so you can see how it compares, and flag it if your figure sits a long way outside that range in either direction. If you're working to a main contractor quote, this is the option to use.

Calculated on current HMRC bands, including the additional dwelling surcharge if you already own a property. It's one of the costs people most often get wrong or forget entirely.

An estimate of what it costs to borrow against the purchase and the build. It's a guide, not a quote. What you actually pay depends on the lender, your track record and the deal itself.

Pulled live from planning.data.gov.uk and PlanWire. Where we can, we filter planning history to your development type, so you're looking at what's been approved for schemes like yours rather than a general local average.

Comparison puts two saved appraisals side by side so you can see the difference between them at a glance rather than flicking between tabs and trying to hold both sets of numbers in your head.

It shows the same figures for each deal, lined up, so the gaps stand out. What it won't do is tell you which one to buy. Two deals can score similarly and still be completely different propositions depending on your cash position, your appetite for planning risk, or how quickly you need to get out.

Avalor is built to help you work out quickly whether something is worth chasing. It isn't a cost plan, a RICS valuation or planning advice. Use it to sift the deals worth looking at properly, then get real quotes and proper advice before you commit to anything.