Reasonable adjustments are changes employers must make to avoid putting an employee with a disability at a substantial disadvantage compared to employees without a disability. They are a legal requirement under the Equality Act 2010.
When an employee (or job applicant) has a 'disability' as defined by the Equality Act: a physical or mental impairment that has a substantial, long-term (12+ months) adverse effect on their ability to carry out normal day-to-day activities. ADHD, autism, depression, anxiety disorders, chronic pain conditions and many others qualify if they meet this definition.
The cost, practicability, effectiveness and disruption to the employer are all considered. What is reasonable for a large employer may not be for a small business. An employer cannot refuse an adjustment solely on cost grounds if it is otherwise practicable.