The 70% Rule Explained: How Smart UK Property Investors Avoid Overpaying
By the Proplytics Research Team
Every experienced property investor has a war story about a deal that looked fantastic on paper but fell apart once the real numbers came in. Overpaying for a renovation project is one of the most common – and most expensive – mistakes in property investment. The 70% rule exists to prevent exactly that.
What Is the 70% Rule?
The 70% rule is a quick-reference formula used by property investors to calculate the maximum offer they should make on a property that needs work. It works like this:
Maximum Offer = (After Repair Value × 70%) – Renovation Costs
The After Repair Value (ARV) is the estimated market value of the property once all renovation work is complete. The 30% margin that the rule builds in is designed to cover your profit, holding costs, finance charges, legal fees, stamp duty, and the inevitable surprises that come with any refurbishment project.
A Worked Example
Say you've identified a three-bedroom terraced house in Sheffield listed at £145,000. Based on comparable sales of recently renovated properties in the same postcode, you estimate the ARV at £210,000. Your builder has quoted £35,000 for a moderate refurbishment – new kitchen, bathroom, redecoration throughout, and some minor structural repairs.
Applying the 70% rule:
- ARV: £210,000
- 70% of ARV: £147,000
- Less renovation costs: £147,000 – £35,000 = £112,000
Your maximum offer should be £112,000. The property is listed at £145,000, which means at asking price this deal doesn't stack up under the rule. You'd need to negotiate a significant discount or walk away.
Why 70% and Not 75% or 80%?
The 30% margin accounts for more than just your profit. It needs to absorb stamp duty, legal fees on purchase and sale, finance costs during the works period, utility costs, estate agent fees on resale, and capital gains tax if applicable.
Once you account for all of that, a 30% margin often leaves a net profit of around 10-15% – a reasonable return for the risk, capital commitment, and time involved. Tighten that margin to 20% and you're left with very little room for error. A single unexpected cost – structural issues hidden behind plasterboard, asbestos removal, a slower-than-expected sale – can wipe out your profit entirely.
When to Adjust the Rule
The 70% rule is a starting point, not a rigid law. There are situations where experienced investors flex it in either direction.
You might tighten to 65% when dealing with higher-risk properties – those with structural concerns, short leases, conservation area restrictions, or in areas with thin transaction volumes where resale could take longer. The additional margin compensates for the added uncertainty.
Conversely, some investors relax to 75% in strong markets with fast-moving stock, where they have reliable builder relationships that reduce cost overruns, or where they're using cash (eliminating finance costs). Buy-to-let investors using the BRRR strategy sometimes accept thinner flip margins because their profit model is built around long-term rental yield and capital growth rather than a single resale event.
The Rule Is Only as Good as Your ARV
Here's where many investors go wrong: they apply the 70% rule correctly but base it on an inaccurate ARV. If your ARV is optimistic by even 10%, your maximum offer calculation is skewed from the start, and the rule's protective margin evaporates.
A robust ARV requires genuine comparable evidence – ideally three to six completed sales of similar properties (same type, similar size, comparable condition post-renovation) within the same postcode sector, sold within the last 24 to 36 months. Online valuation tools can give you a starting point, but they rarely account for the specific condition, layout, or micro-location factors that drive real sale prices.
This is precisely why we build every Proplytics report around verified comparable sales data, with each comp individually assessed for relevance rather than relying on automated estimates.
Common Mistakes to Avoid
Underestimating renovation costs. Always get at least two builder quotes before running your numbers, and add a contingency of 10-15% on top. Cosmetic refreshes have a habit of uncovering deeper problems once work begins.
Ignoring holding costs. If you're using bridging finance at 0.75% per month and the project takes two months longer than planned, that's a meaningful hit to your margin.
Using asking prices as comps. Only completed sale prices from the Land Registry count as genuine comparables. What a neighbouring property is listed for tells you what the seller hopes to achieve, not what the market will actually pay.
Forgetting the exit. A property might pass the 70% rule beautifully, but if it's in an area with low transaction volumes, you could be waiting months for a buyer – accumulating costs the whole time.
The Bottom Line
The 70% rule won't guarantee a profitable deal, but it will consistently steer you away from bad ones. It forces discipline at the point where enthusiasm most often overrides judgement – the moment you're about to make an offer. Use it as your first filter, build it on solid ARV evidence, and respect the margin it creates. Your portfolio will thank you for it.
At Proplytics, every report includes a 70% rule assessment with a clear pass/fail indicator and maximum offer calculation, built on verified comparable sales. Explore our services to see how we can support your next investment decision.