Model/recommendations/Costs.py

class Costs:
    """
    A class to calculate the costs associated with construction works,
    specifically focusing on cavity wall insulation.
    It includes contingency, preliminaries, profit margin, and VAT calculations.

    As a sense check, there is a useful article from checkatrade on retrofitting walls and expected costs:
    https://www.checkatrade.com/blog/cost-guides/retrofit-insulation-cost/
    """

    # Contingency is a percentage of the total cost of the work and covers unforseen expenses
    # We assume a conservative 10% contingency for all works which is a rate defined by SPONs
    CONTINGENCY = 0.1

    # Preliminaries are a percentage of the total cost of the work and covers the cost of site-specific costs
    # such as site preparation, safety measures and project management. This rate can vary but we'll assume a 10%
    # rate, on the total cost before VAT, as recommended by SPONs
    PRELIMINARIES = 0.1

    VAT_RATE = 0.2
    PROFIT_MARGIN = 0.15

    def __init__(self, property_instance):
        """
        Initializes the Costs class with a property instance.

        :param property_instance: Instance of a Property class containing relevant details like wall area.
        """
        if not hasattr(property_instance, 'insulation_wall_area'):
            raise ValueError("Property instance must have an 'insulation_wall_area' attribute")
        self.property = property_instance

    def cavity_wall_insulation(self, material):
        """
        Calculates the total cost for cavity wall insulation based on material and labor costs,
        including contingency, preliminaries, profit, and VAT.

        :return: A dictionary containing detailed cost breakdown.
        """
        # Cost per m2
        material = {
            "description": "Crown Dritherm Cavity Slab 37 (Thermal conductivity 0.037 W/mK) glass fibre batt or other "
                           "equal; as full or partial cavity fill; including cutting and fitting around wall ties and "
                           "retaining discs",
            "depth": 75,
            "thermal_conductivity": 0.037,
            "prime_cost": 5.17,
            "material_cost": 5.62,
            "labour_cost": 2.25,
            "labour_hours": 0.13
        }

        material_cost_per_m2 = material["material_cost"]
        wall_area = self.property.insulation_wall_area

        # This is the amount of material required in m3, assuming a standard 75mm depth
        volume = 0.075 * wall_area

        base_material_cost = material_cost_per_m2 * wall_area
        labour_cost = material["labour_cost"] * wall_area

        subtotal_before_profit = base_material_cost + labour_cost

        contingency_cost = subtotal_before_profit * self.CONTINGENCY
        preliminaries_cost = subtotal_before_profit * self.PRELIMINARIES
        profit_cost = subtotal_before_profit * self.PROFIT_MARGIN

        subtotal_before_vat = subtotal_before_profit + contingency_cost + preliminaries_cost + profit_cost

        vat_cost = subtotal_before_vat * self.VAT_RATE

        total_cost = subtotal_before_vat + vat_cost

        labour_hours = material["labour_hours"] * wall_area

        return {
            "total": total_cost,
            "subtotal": subtotal_before_vat,
            "vat": vat_cost,
            "contingency": contingency_cost,
            "preliminaries": preliminaries_cost,
            "material": base_material_cost,
            "profit": profit_cost,
            "labour_hours": labour_hours
        }

    def loft_insulation(self, material):
        """
        Calculates the total cost for cavity wall insulation based on material and labor costs,
        including contingency, preliminaries, profit, and VAT.

        :return: A dictionary containing detailed cost breakdown.
        """
        # Cost per m2
        # material = {
        #     "description": "Crown Loft Roll 44 glass fibre roll",
        #     "depth": 270,
        #     "thermal_conductivity": 0.044,
        #     "prime_cost": None,
        #     "material_cost": 5.91938,
        #     "labour_cost": 1.96,
        #     "labour_hours": 0.11
        # }

        material_cost_per_m2 = material["material_cost"]
        floor_area = self.property.floor_area

        base_material_cost = material_cost_per_m2 * floor_area
        labour_cost = material["labour_cost"] * floor_area

        subtotal_before_profit = base_material_cost + labour_cost

        contingency_cost = subtotal_before_profit * self.CONTINGENCY
        preliminaries_cost = subtotal_before_profit * self.PRELIMINARIES
        profit_cost = subtotal_before_profit * self.PROFIT_MARGIN

        subtotal_before_vat = subtotal_before_profit + contingency_cost + preliminaries_cost + profit_cost

        vat_cost = subtotal_before_vat * self.VAT_RATE

        total_cost = subtotal_before_vat + vat_cost

        labour_hours = material["labour_hours"] * floor_area

        return {
            "total": total_cost,
            "subtotal": subtotal_before_vat,
            "vat": vat_cost,
            "contingency": contingency_cost,
            "preliminaries": preliminaries_cost,
            "material": base_material_cost,
            "profit": profit_cost,
            "labour_hours": labour_hours
        }