I would recommend readng
Maintenance and accommodationsection of the Entry Clearance Guidance (ECG) available on the UKVisas website.
You will need the following (from the sponsor):
1. A cover letter from sponsor stating his relationship with the applicant(s) and that the sponsor is ready to accommodate and / or take the responsibility of maintaining the applicant(s) during his/her visit to UK. If accomodating the applicant(s) in your own residence, this letter should include the following details / information:-
a) Size of property (e.g.- 2 bedroom house, 3 bedroom flat, studio etc.).
b) Number of people currently living there.
c) If possible, confirmation of a dedicated room for the applicant(s). Mandatory when couples are to be accommodated.
d) whether property is rented or owned.
(If the property is rented, include in the list below a copy of the tenancy agreement and permission from landlord allowing the sponsor to accommodate extra persons).
2. Photocopy of sponsor's proof of income / salary slips / bank statements showing income credit and availability of funds (if sponsor is taking the responsibility of maintenance) - covering last 4-6 months.
3a. Photocopy of council tax bill for current year (gives the ECO an idea of size of property).
or
3b. Photocopy of utility bills - covering last 4-6 months.
4. Copy of personal details and *current* visa pages of sponsor's passport.
It is important to provide documentary evidence to prove that EC applicant(s) has close ties with home country and will return at the end of the visit (e.g.- employment letter, savings held or investments made, home ownership, evidence of govt. pension etc. etc.).
Letter from sponsor - original.
UK documents (sponsor's) - original or photocopies.
Passport (sponsor's) - photocopies.
Home country documents (applicant's) - in original.
Hope this helps.
regards