The San Martín Scale, a 95-item Likert scale questionnaire( completed by a third-party respondent has been validated and may meet your requirements. Herewith the abstract . Kind regards. Sherva

Res Dev Disabil. 2014 Jan;35(1):75-86. doi: 10.1016/j.ridd.2013.10.025. Epub 2013 Nov 15.

Measuring quality of life in people with intellectual and multiple disabilities: validation of the San Martín scale.

Verdugo MA1, Gómez LE, Arias B, Navas P, Schalock RL.

Author information

Abstract

Although there are numerous quality of life instruments in the, field of intellectual disability, most of them are addressed to those, people with the highest levels of functioning, while only a few are, suitable for people with the lowest levels (i.e., people with profound, and severe intellectual disabilities, or people with intellectual and, developmental disabilities and other significant medical conditions or, disabilities). This study provides reliability and validity evidence of, the San Martín Scale, a 95-item Likert scale questionnaire that is, completed by a third-party respondent. The validation sample was composed, of 1770 people from Spain with intellectual and developmental, disabilities that showed extensive or pervasive support needs (8.7% had, mild intellectual disability, 28.25% moderate, 41.6% severe, and 21.4%, profound). The age of the participants ranged between 16 and 77 years old, (M=7.78; SD=12.32). The results suggested that the eight quality of, life domains assessed on the scale are reliable (Cronbach's alpha ranging, from .821 to .933). Confirmatory Factor Analyses provided construct, validity evidences related to the internal structure of the San Martín, Scale, and indicated that the eight first-order factor solution provided, the best fit to the data over unidimensional and hierarchical solutions. Implications of these findings and guidelines for further research are, discussed.

Hello Marilena

The full text of the above paper is available on ResearchGate:

https://www.researchgate.net/publication/258640894_Measuring_quality_of_life_in_people_with_intellectual_and_multiple_disabilities_Validation_of_the_San_Martin_scale

I wondered if this was of any help, as the 'self determination' suggests choices:

Nota, L., Ferrari, L., Soresi, S., & Wehmeyer, M. (2007). Self‐determination, social abilities and the quality of life of people with intellectual disability. Journal of Intellectual Disability Research, 51(11), 850-865.

https://www.researchgate.net/publication/5933566 

However, I was unable to access the instrument as referenced by the authors.

This is by the same researchers:

Nota, L., Soresi, S., & Perry, J. (2006). Quality of life in adults with an intellectual disability: the evaluation of quality of life instrument. Journal of Intellectual Disability Research, 50(5), 371-385.

https://www.researchgate.net/publication/7150606_Quality_of_life_in_adults_with_an_intellectual_disability_The_Evaluation_of_Quality_of_Life_Instrument

Very best wishes,

Mary

Article Self-Determination, Social Abilities and the Quality of Life...

Article Quality of life in adults with an intellectual disability: T...

Article Measuring quality of life in people with intellectual and mu...

Thank you very much for your help!!!! Although i prefer something that can be completed from the person with ID rather than proxy responses, if you have something in mind.. 

The field of intellectual disability (ID) is strongly influenced by the Quality of Life paradigm .This Quality of Life framework supports the equality of persons, which is reflected in concepts such as self-determination, emancipation, inclusion, and empowerment.  The approach to the measurement of the Quality of Life can be characterized by the following:

(a)its multidimensional nature involving core domains and indicators

(b)the use of methodological pluralism that includes the use of subjective and objective measures

(c)the incorporation of a systems perspective that captures the multiple environments impacting people at the micro-, meso-, and macrosystems levels

(d)the increased involvement of persons with ID in the design and implementation processes.

Focus groups were organized to gather qualitative data. The first focus group consisted of four mothers, two fathers, and one stepmother, who were all closely involved with their family members with ID. The second focus group consisted of professionals who were employed in the three facilities represented in this research: three staff members of both the psychiatric centers and four of the unit for people with ID. The selection of the professional workers/staff was based on age, gender, years of experience (from 1 up to 30 years of experience) and their level of education. The staff members of the psychiatric centers were psychiatric nurses or educational specialists.  These of the service center for people with ID were educational specialists and one social worker.

Hello Marilena,

I was wondering if this was appropriate:

J. van Loon, G. Van Hove, R.L. Schalock, C. Claes

Personal Outcomes Scale: A Scale to Assess an Individual's Quality of Life

Stichting Arduin and Gent: University of Gent, Middelburg (2008)

I found it referenced in:

Balboni, G., Coscarelli, A., Giunti, G., & Schalock, R. L. (2013). The assessment of the quality of life of adults with intellectual disability: The use of self-report and report of others assessment strategies. Research in developmental disabilities, 34(11), 4248-4254.

https://www.researchgate.net/publication/257647174_The_assessment_of_the_quality_of_life_of_adults_with_intellectual_disability_The_use_of_self-report_and_report_of_others_assessment_strategies

But I have not managed to find an English or Greek version, although it has been translated into Spanish and Portuguese.

From the paper by Balboni et al. it sounds suitable for self-completion. Maybe the authors could provide you with a copy?

Very best wishes,

Mary

Article The assessment of the quality of life of adults with intelle...