There are two sets of criteria for each mortgage application. The first set refers to the property itself, and the second set to your personal circumstances.
In terms of the property:
Studio flat size - a micro home is an efficient, smaller space designed for compact living. Many lenders will not mortgage any flat less than 30 m2, although that can vary. Studios without a separate kitchen area, or where the entrance is adjacent to an outdoor walkway, can also be harder to loan against.
Neighbouring properties - if the studio is above, or next to a commercial business, it becomes higher risk. Problems such as noise, odours and fire risk all make a property riskier as it would be harder to sell in a repossession scenario.
Construction - properties over seven storeys high can also be an issue. Likewise, a studio in a block built from concrete, or another non-standard material, can present a higher risk.
Leasehold - nearly all flats are purchased leasehold, and the length of time remaining will be a factor. Long leases of 90+ years are lower risk, with anything under 60 years a little more problematic.