Works needed to provide access for a person with a disability to an existing building are generally exempted development, provided the ramp and any associated handrails are for that purpose and remain within the curtilage.
The exemption is intended to remove barriers to accessibility works. In practice, a ramp serving the front door of a house, with associated handrails and a modest landing, is unlikely to require permission.
Where the works involve a protected structure, extend beyond the curtilage onto public footpath, or form part of a larger extension, permission may still be required. A section 5 declaration removes doubt.