If a landlord proposes to carry out works that will cost any one leaseholder more than £250 or is looking to enter into a long term qualifying contract (Longer than 12 months) that will cost any one leaseholder more than £100, the landlord is required to go through a consultation procedure under section 20 of the Landlord and Tenant Act 1985.
A section 20 process is made up of three distinct stages
1) Notice of Intention to Carry Out Works” upon all lessees. The Notice must generally describe the proposed works, state the reasons for considering the proposed works, and invite leaseholders to make written observations within 30 days
2) At the expiration of the 30 day consultation period, at least two estimates should be obtained:
“Notice to Accompany the Statement of Estimates” must also be served in conjunction with the Statement of Estimates, which sets out the hours and place where details of the estimates may be inspected, inviting lessees to make written observations on the estimates within 30 days, specifying the address to which those observations should be sent.
3) Award of contract
If at the expiration of the consultation period, the chosen contractor did not provide the lowest estimate, then a “Notice of Reasons” must be served upon all lessees
Section 20 of the Landlord and Tenants is the subject of a great deal of case law and is a subject of discussion in it's own right
For further reading see https://www.lease-advice.org/advice-guide/section-20-consultation-private-landlords-resident-management-companies-agents/