People usually call having already decided which one they want, and about a third of the time they have picked the one that does not solve their actual problem.
Start with what is wrong with the house
Not with what you fancy building. If the complaint is there is nowhere to put anybody, you need a bedroom, and a loft conversion is almost certainly the answer. If the complaint is we all end up standing in a tiny kitchen, no loft conversion in the world fixes that.
Cost per square metre
A dormer loft giving you a double bedroom and an ensuite frequently lands under what a two storey extension would cost for the same rooms, because you are not paying for foundations or a new roof.
The comparison that matters
Garden
A loft costs you none. An extension costs you a chunk of it, and on a small plot that matters more than people expect until it is gone.
Planning
Both are frequently permitted development. Two storey extensions and mansard lofts are the two that usually need a full application.
Disruption
A loft is surprisingly liveable, because most of the work happens above your ceilings and is reached from outside. An extension is noisier and dustier at the start but the breakthrough into the house comes right at the end, so you keep living in it too.
Timescale
Loft conversions run 4 to 14 weeks depending on type. Single storey extensions 8 to 12 weeks, two storey 14 to 20.
Effect on the listing
A loft conversion changes the bedroom count, which is what estate agents and search filters run on. An extension makes existing rooms better without changing that number. Both add value. The loft is more visible to a buyer scrolling a property site.
When the answer is both
On a lot of Nottingham semis the honest answer is that you want a rear extension now and a loft in three years, or the other way around. If that is likely, tell your builder at the start. Where the soil stack runs, where you leave the consumer unit and how the roof is structured all get decided during the first job, and a bit of thought then saves real money later.
And when the answer is neither
If the house is fundamentally the wrong shape or the wrong size, no amount of building fixes it and moving is the right call. We will tell you if we think that is your situation. We would rather lose a job than take 50,000 pounds off somebody to build something that does not solve their problem.
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