A loft conversion looks like the cheapest way to gain space. Sometimes it is. But the answer comes before any sketch, not after.

Will the structure take it?
A 1960s block was not calculated for an extra level. A structural survey says whether it is possible and on what terms. It is the first cost and it cannot be skipped.

The neighbours agreement
Work on common parts requires the consent of the owners association and, depending on the case, of directly affected neighbours. It is obtained before filing, not during.

The height limit for the area
If the local plan caps the height at what already exists, the conversion is impossible whatever the structure says.

What changes in the stairwell
An extra level may require fire escape upgrades. Above four storeys, fire brigade approval becomes mandatory and changes the cost.

How long it takes
Survey one month, design two, permit four to nine. Construction, three or four months. Anyone promising less has never done one.