Net cost comparison: extending vs buying a bigger home

What the Two Options Actually Cost

A loft conversion averages 40-80k adding 30-60k to the home's value. A two-storey side extension runs 75-150k, adding roughly 70-80% of spend to the home's value. Net extension cost is typically 10-30k — less than a single moving round trip often costs in property transfer tax and fees.

The True Cost of Moving

A move from 350k to 500k home in carries roughly 12,500 in property transfer tax, 2-4k estate agent fees on the sale, 1,500-3,000 in legal fees, plus removals and incidentals. The full round-trip cost runs 20-30k before the mortgage rises to cover the extra 150k. An equivalent extension might add just as much floor space at one-third of the total cost.

Factors Outside the Formula

Area (catchment, transport, neighbours) is the single biggest variable a calculator can't capture. Extensions don't change the postcode. Building disruption (3-6 months of builders) vs move stress (one painful weekend) are also trade-offs money can't fully quantify. This tool handles the financial side; the lifestyle call is yours.

Quick example

With current home value of 350,000 and extension build cost of 60,000 (plus expected value uplift from extension of 50,000 and alternative new home price of 500,000), the result is 160,500.00. Change any figure and watch the output shift — it's often more useful to see the pattern than to memorise the formula.

Which inputs matter most

You enter Current Home Value, Extension Build Cost, Expected Value Uplift from Extension, Alternative New Home Price, and Moving + Legal + Agent Fees. Two inputs usually tip the answer one way or the other. Identify which ones matter most by flipping each value past a round threshold and watching whether the winning option changes.

What's happening under the hood

Net extension cost equals build cost minus expected value uplift. Total move cost equals new-home-minus-current-home price gap plus moving fees plus property transfer tax. The option with lower cost wins. Results are estimates for illustration purposes only. The formula is listed in full below. If the number looks off, you can retrace the calculation by hand — that's the point of showing the working.

Reading payback vs outright cost

Payback tells you when you're break-even, not whether the purchase is a good idea. A short payback on something you barely use is still a loss. Pair the number with an honest count of expected usage.

What this doesn't capture

Purchase decisions rarely come down to payback alone. Reliability, time saved, enjoyment, and alternatives outside the calculation all matter. The figure gives you the money side cleanly so you can weigh it against everything else honestly.