Hi Hugo

Depending on the Odoo version and the possibility to install third party apps, a simple and efficient solution already used by the community for a long time would be for you to install "Sale product set": https://odoo-community.org/shop/product/sale-product-set-2334#attr=9582

BR

Ricardo Gross

You could define these products as KITs (BOM), so the qty / multiples would be applied only on the specific KIT. 

This would mean you could have 20 m² terrace along with 45 mts of edge cutting on the same SO.

However, the limitation with standard Odoo would be the KIT product would be 1 SO line and NOT 1 line per component. The challenge would be if there is need to invoice individual components having different tax rates.

If the need for component is for information purposes only, you could always use the sales description field to put down the component and qty information.

Or apply any small customization that would break down the KIT BOM line in SO to individual component lines. (not being a developer am unable to help here)!

A small development might resolve this, here is a prototype:

1. Create a custom field on the Sale Order so Users can enter a line multiplier and add it to the UI:

2. On create or on change of the sale order line, update the quantity based on the multiplier (this code works on change but it might be better to do this on create only):

This sample code comes from a custom field (to validate the prototype) but would be implemented via a module:

COMPUTE:

for record in self:
    record['x_onchange_trigger'] = False
        if record.product_uom_qty > 0:
            multiplier = record.order_id.x_multiplier
            if multiplier > 0:
                record['product_uom_qty'] = record.order_id.x_multiplier * record.product_uom_qty

DEPENDENCIES:
product_uom_qty

3. When the multiplier is 0, everything will have a quantity of 1 and can be edited to any other quantity:

4. When the multiplier is > 1, everything will have quantity to match the multiplier (and for this prototype - each time the User enters a quantity it will be multiplied by 45 - but you can modify this by leveraging on create instead of on change):

This is just a prototype, the User would have to change 45 back to 0 or to another multiplier if they wanted the next things they add to the Sales Order to not use 45 as the quantity.  It may or may not work for your case and there could be a better design/code.