What benefits is your fiance on?
Usually, you have to show that the UK citizen is earning £18,600 or more per year, in order for you to qualify for a spousal visa.
However, if she is receiving certain benefits, she will be exempt from the income requirement of £18,600 and instead you will only have to show that between you, you have enough money to live off without you (the US citizen) claiming benefits (essentially that you have about £113 left over each week after paying any rent and council tax, but before other expenses).
The benefits that would make her exempt from meeting the income requirement are:
- Carer‟s Allowance.
- Disability Living Allowance.
- Severe Disablement Allowance.
- Industrial Injuries Disablement Benefit.
- Attendance Allowance.
- Personal Independence Payment.
If she is receiving any of these, you may be able to qualify for the spousal visa easily using her benefit income and your pension income.
If not, you will have to meet one of the income requirements:
- Category A or B (her employment): She earns at least £18,600 per year in the UK
- Category C: Non-employment income of £18,600 per year between you (i.e. rent from property you own, stocks/shares/dividends income etc.)
- Category D: You have £62,500 or more in savings and have held this amount for at least 6 months
- Category E: Pension income - your US pension is at least £18,600 per year
- Category F or G: She is self-employed in the UK and has earned an average of £18,600 over the last 1, or the last 2 financial years.
See here for information about the income requirements:
http://www.ukba.homeoffice.gov.uk/sitecontent/documents/policyandlaw/IDIs/chp8-annex/section-FM-1.7.pdf?view=Binary