The age of the house and its original roof construction "style" are the main "yes" or "No" answers to this question. I am a Building Regualtions surveyor, and a loft conversion is "nearly always possible" but the costs can be for some original house & roof types "not worth while".
Loft conversions
Converting the space under the roof of your home can be a cost effective way of creating extra bedrooms or living areas. But not all lofts are suitable for conversion. As a first step you should check the following:
Height: Is there enough room in your loft to stand comfortably? For your loft to be classified as a bedroom once converted you will ideally need a minimum of 2m headroom – but remember this will be after new floors, beams and panelling have been fitted, which may take up another 300mm of headroom.
Floorspace: Is the floor area large enough to make a useable room?
Utilities: Would any chimneys, tanks, pipes or services require moving to create a usable space?
If your uncoverted loft space is too low, or too small or has too many utilities it may still be possible to convert it, but it will be much more complex (and expensive). And you should talk to your local building control team for advice.
Get professional advice
Any loft conversion is a complex project, so you should get professional plans drawn up by an architect or building designer and get them approved by your local council’s building control team before starting work - you may also need Planning Permission so it's always best to check with your Local Authority Planning Department. The plans should cover:
Roof – structure (loading, supports and beams), materials, insulation and ventilation
Access – stairs (angle, width and height), fire escape, handrails and bannisters
Floors and walls – structure and strengthening, insulation and sound proofing
Electrics, power and heating
Windows and doors
Fire safety – escape routes and smoke detectors.
Bathrooms – connection to water supply and drains, ventilation
Advice is available on the LABC Website (The National "Local Authority Building Control") See www.labc.co.uk and a video at www.labc.co.uk/advice-building-projects/homeowners/building-a-loft-conversion
There are also many other "Building Regualtion" websites.
John Bone, C.Build.E MCABE, BSc Hons