Many homeowners focus entirely on comparing window styles and quotes when planning a replacement — overlooking one crucial step: under Buildings Department regulations, replacing aluminium windows is classified as minor works and must be declared before construction begins. Failure to comply can result in legal liability. What are the three classes of minor works? Which one applies to your window replacement? And how complicated is the process? This guide breaks it all down so you're fully prepared.
Why Does Window Replacement Require a Declaration? The Minor Works Scheme Explained
Most residential aluminium window replacements in Hong Kong fall under the Building (Minor Works) Regulation. The Buildings Department introduced the Minor Works Supervision System to ensure that works affecting a building's external walls are carried out by qualified persons, safeguarding public safety.
The scheme categorises common home building works by complexity and risk into Class I, II, and III, each with different declaration requirements and procedures.
A standard residential aluminium window replacement typically falls under Class II Minor Works — meaning the works must be carried out by a licensed Registered Minor Works Contractor and declared to the Buildings Department both before and after construction.
If only fittings such as window hinges or handles are being replaced, rather than the full window unit, this generally counts as routine maintenance and may not trigger the minor works declaration requirement — though this should always be confirmed by a qualified person.
The Three Classes of Minor Works: Which Class Applies to Your Window Replacement?
The Buildings Department classifies minor works into three classes by the nature, scale, complexity and safety risk of each project. Note: dimensions such as 50cm, 1.1m, 3.5m or 100m are merely technical thresholds within individual project items — they are not the basis for classification, so do not judge the class by dimensions alone.
| Class | Correct Definition | Who Is Responsible | Declaration Before Works | After Completion |
|---|---|---|---|---|
| Class I | More complex, higher-risk minor works | Must appoint a prescribed building professional (e.g. an Authorised Person) to design and supervise — possibly also a structural/geotechnical engineer — with a Prescribed Registered Contractor carrying out the works | Yes — submit MW01, plans, project details and photos at least 7 days before commencement | Submit MW02, record plans and photos within 14 days |
| Class II | Medium complexity and risk minor works (full aluminium window replacement falls here) | Appoint a Prescribed Registered Contractor to prepare documents, supervise and carry out the works; generally no separate Authorised Person required | Yes — submit MW03, plans/details and photos at least 7 days before commencement | Submit MW04, record plans and photos within 14 days |
| Class III | Simpler, lower-risk minor works | Appoint a Prescribed Registered Contractor to carry out the works; some items may be done by a qualified individual contractor | Not required before commencement | Submit MW05, works description or record plans and photos within 14 days |
For private flat and HOS flat owners, Class II Minor Works is the most common category — referring to a full replacement of the entire aluminium window unit. It is important to note that "window replacement" here means replacing the complete window frame and glazing as a unit, not simply replacing the glass panels or individual fittings.
If you are comparing aluminium window replacement costs, understanding the minor works declaration requirement will help you verify whether a contractor's quote already includes the declaration service, avoiding unexpected charges later.
Class II Minor Works: A Step-by-Step Guide to the Window Replacement Declaration
Many homeowners worry the paperwork is complicated. In practice, once you appoint a licensed contractor to handle it on your behalf, the process is straightforward. Here is the standard procedure:
Step 1: Appoint a Licensed Registered Minor Works Contractor
The homeowner must appoint a contractor holding a Buildings Department-recognised licence (such as MWC 316/2021) to carry out the works and manage the declaration. The contractor will submit a Commencement Notice (Form MW03) to the Buildings Department on your behalf, at least 7 days before works commence.
Step 2: Submit the Commencement Notice and Await Acknowledgement
After the contractor submits the Commencement Notice, the Buildings Department acknowledges receipt, after which construction may legally proceed.
Step 3: Carry Out the Window Replacement Works
During the works, the licensed contractor must supervise the installation to ensure compliance with specifications — including the use of approved materials and correct installation methods.
Step 4: Submit Completion Documents
Once the works are finished, the contractor must submit a Completion Certificate (Form MW04) to the Buildings Department within 14 days of completion. This document serves as an important record for owners' corporations, insurance claims, and property transactions — keep a copy for your own records.
Step 5: Receive Completion Confirmation
Once the Buildings Department confirms receipt of the completed documents, the lawful window replacement and declaration process is fully concluded.
Real Case: Full Window Replacement with Declaration in Kwun Tong
A Kwun Tong homeowner contacted Amgen to carry out a full window replacement for a private flat over 20 years old. During the site inspection, the existing aluminium frames showed multiple signs of deformation, and the weatherproof silicone seals had become hard and contracted, causing water ingress during the typhoon season.
As all 12 windows throughout the flat required complete replacement, the works met the requirements for a Class II Minor Works declaration. Amgen handled all the paperwork on the homeowner's behalf — including the Commencement Notice and Completion Statement — so the owner did not need to deal with any administrative procedures personally. The full process from declaration to completion took approximately two weeks, and the Buildings Department received all documents upon completion.
Frequently Asked Questions
Q: What happens if I replace windows without filing a declaration?
A: Under the Buildings Ordinance, carrying out building works classified as minor works without proper authorisation can expose both the homeowner and the contractor to legal liability, including fines or a requirement to reinstate the original condition. Always confirm with your contractor that the declaration has been filed before any works begin.
Q: Is the declaration process different for HOS flats compared to private flats?
A: The two are overseen by different authorities. Private flats are regulated by the Buildings Department (BD) under the Buildings Ordinance, whereas minor works in public rental housing, HOS flats and Tenants Purchase Scheme units under the Housing Authority are handled by the Independent Checking Unit (ICU) of the Office of the Permanent Secretary for Housing. HOS flat owners should check the applicable procedure and any required consent with their estate office or management office before replacing windows.
Q: Do I need to declare even if I'm only replacing a few windows?
A: The minor works declaration requirement is based on the nature of the works, not the number of windows replaced. Any full replacement of an entire window unit qualifies as minor works. Replacing individual fittings such as hinges or handles is generally considered maintenance, but please confirm with a qualified person for your specific situation.
Q: Can a tenant arrange window replacement and handle the declaration independently?
A: The declaration responsibility lies with the property owner. Tenants who wish to replace windows should first obtain written consent from their landlord, after which the landlord should appoint a licensed contractor to carry out the works and handle all required declarations.
What are the Buildings Department's requirements for aluminium window specifications?
Newly replaced aluminium windows must comply with the Buildings Ordinance and the relevant codes of practice, covering wind-load resistance, glass thickness, opening and balustrade height and other safety requirements, and must be confirmed by a qualified person. Because full window replacement is alteration work involving the wall structure, it is classified as Class II Minor Works and must comply with the Class II declaration (commencement form MW03, completion form MW04). The exact Buildings Department specifications should be assessed by a licensed contractor based on your unit.
Commission a Licensed Contractor for a Compliant Window Replacement
Window replacement is an important decision involving external wall works, and a proper declaration is an essential step to protect your interests. If you are planning a replacement, our aluminium window style guide can help you choose the most suitable window type for your home. If you also have a mandatory window inspection requirement, Amgen can arrange both together to save you administrative time.
Amgen Windows holds licence MWC 316/2021 and is fully qualified to handle Class II Minor Works declarations. We provide complete documentation records upon project completion. For aluminium window replacement, a free assessment or enquiry, please contact us via WhatsApp at 6586 3435.
