Should new siding be installed over existing siding, or should the old material be removed?
The existing surface is uneven or damaged
Ask how the contractor will establish a suitable supporting surface. Vinyl installation guidance recognizes that leveling or removal of uneven siding may be necessary. Covering a distorted surface without resolving it can carry the problem into the new finish.
Should new siding be installed over existing siding, or should the old material be removed?
The existing wall has a moisture history
Request an investigation plan before choosing concealment. The contractor should explain how the drainage plane and flashing will be assessed or restored, including any access needed. A claim that the new siding is water-resistant does not answer whether concealed water can escape.
Should new siding be installed over existing siding, or should the old material be removed?
An overlay changes the wall thickness
Confirm the proposed treatment at trim, openings and penetrations. The added layer may require revised details rather than simply extending the same cosmetic trim. Ask which manufacturer's approved assembly is being followed and how attachment reaches the intended supporting material.
Should new siding be installed over existing siding, or should the old material be removed?
Compare the bids on the same basis
Provide existing product records, photographs, past leak information and drawings of planned changes. Ask a siding contractor for separate line items covering preparation, concealed repairs, flashing and disposal. Manufacturer support can clarify the proposed assembly; the local building authority can clarify approvals, while a hazardous-materials professional handles suspect old material.
Should new siding be installed over existing siding, or should the old material be removed?
What remains a property decision
This answer does not approve all overlays or require all old siding to be removed. Hidden condition, product compatibility and local requirements remain unverified. A lower initial quote is not a complete comparison unless access, weather protection and the resulting wall details are included.