Use this calculator to estimate the energy performance of your proposed extension based on key fabric U-values and areas. This provides a simplified indication of compliance and potential heating demand, but is not a substitute for a full SAP calculation by a qualified assessor.
New Fabric Elements
Enter the area and U-value for each new external element of your extension. U-values represent how well a material insulates; lower is better.
Heating System & Location
Typical UK average is 2500-3000. Higher values mean colder climate.
.calculator-container {
font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
background-color: #f9f9f9;
padding: 20px;
border-radius: 8px;
box-shadow: 0 2px 10px rgba(0, 0, 0, 0.1);
max-width: 700px;
margin: 20px auto;
color: #333;
}
.calculator-container h2, .calculator-container h3 {
color: #0056b3;
border-bottom: 2px solid #e0e0e0;
padding-bottom: 10px;
margin-top: 25px;
}
.calculator-container p {
font-size: 0.95em;
line-height: 1.6;
margin-bottom: 15px;
}
.calc-input-group {
margin-bottom: 15px;
display: flex;
flex-direction: column;
}
.calc-input-group label {
margin-bottom: 5px;
font-weight: bold;
color: #555;
font-size: 0.9em;
}
.calc-input-group input[type="number"] {
padding: 10px;
border: 1px solid #ccc;
border-radius: 4px;
font-size: 1em;
width: 100%;
box-sizing: border-box;
}
.calc-input-group .description {
font-size: 0.85em;
color: #777;
margin-top: 5px;
}
.calculator-container button {
background-color: #007bff;
color: white;
padding: 12px 20px;
border: none;
border-radius: 5px;
cursor: pointer;
font-size: 1.1em;
margin-top: 20px;
transition: background-color 0.3s ease;
width: 100%;
box-sizing: border-box;
}
.calculator-container button:hover {
background-color: #0056b3;
}
.calculator-result {
margin-top: 30px;
padding: 20px;
background-color: #eaf6ff;
border: 1px solid #b3d9ff;
border-radius: 8px;
font-size: 1.1em;
color: #004085;
}
.calculator-result h3 {
color: #0056b3;
margin-top: 0;
border-bottom: none;
padding-bottom: 0;
}
.calculator-result p {
margin-bottom: 10px;
}
.calculator-result ul {
list-style-type: none;
padding: 0;
}
.calculator-result ul li {
background-color: #d9edf7;
margin-bottom: 5px;
padding: 8px 12px;
border-left: 4px solid #2196f3;
border-radius: 4px;
}
.calculator-result .pass {
color: #28a745;
font-weight: bold;
}
.calculator-result .fail {
color: #dc3545;
font-weight: bold;
}
.calculator-result .good {
color: #28a745;
font-weight: bold;
}
.calculator-result .average {
color: #ffc107;
font-weight: bold;
}
.calculator-result .poor {
color: #dc3545;
font-weight: bold;
}
function calculateSapExtension() {
// Get input values
var extensionFloorArea = parseFloat(document.getElementById('extensionFloorArea').value);
var newWallArea = parseFloat(document.getElementById('newWallArea').value);
var newWallUValue = parseFloat(document.getElementById('newWallUValue').value);
var newRoofArea = parseFloat(document.getElementById('newRoofArea').value);
var newRoofUValue = parseFloat(document.getElementById('newRoofUValue').value);
var newFloorArea = parseFloat(document.getElementById('newFloorArea').value);
var newFloorUValue = parseFloat(document.getElementById('newFloorUValue').value);
var newWindowArea = parseFloat(document.getElementById('newWindowArea').value);
var newWindowUValue = parseFloat(document.getElementById('newWindowUValue').value);
var newDoorArea = parseFloat(document.getElementById('newDoorArea').value);
var newDoorUValue = parseFloat(document.getElementById('newDoorUValue').value);
var heatingEfficiency = parseFloat(document.getElementById('heatingEfficiency').value);
var heatingDegreeDays = parseFloat(document.getElementById('heatingDegreeDays').value);
// Define typical target U-values for compliance (Building Regulations Part L1B, England & Wales)
var targetWallUValue = 0.28; // W/m²K
var targetRoofUValue = 0.15; // W/m²K
var targetFloorUValue = 0.18; // W/m²K
var targetWindowUValue = 1.4; // W/m²K
var targetDoorUValue = 1.4; // W/m²K (often 1.6, but 1.4 is good for overall performance)
// Validate inputs
if (isNaN(extensionFloorArea) || isNaN(newWallArea) || isNaN(newWallUValue) ||
isNaN(newRoofArea) || isNaN(newRoofUValue) || isNaN(newFloorArea) ||
isNaN(newFloorUValue) || isNaN(newWindowArea) || isNaN(newWindowUValue) ||
isNaN(newDoorArea) || isNaN(newDoorUValue) || isNaN(heatingEfficiency) ||
isNaN(heatingDegreeDays) ||
extensionFloorArea < 0 || newWallArea < 0 || newWallUValue <= 0 ||
newRoofArea < 0 || newRoofUValue <= 0 || newFloorArea < 0 ||
newFloorUValue <= 0 || newWindowArea < 0 || newWindowUValue <= 0 ||
newDoorArea < 0 || newDoorUValue <= 0 || heatingEfficiency 100 ||
heatingDegreeDays <= 0) {
document.getElementById('result').innerHTML = 'Please enter valid positive numbers for all fields. Heating efficiency must be between 1 and 100.';
return;
}
// 1. Calculate Total Fabric Heat Loss (W/K)
var wallHeatLoss = newWallArea * newWallUValue;
var roofHeatLoss = newRoofArea * newRoofUValue;
var floorHeatLoss = newFloorArea * newFloorUValue;
var windowHeatLoss = newWindowArea * newWindowUValue;
var doorHeatLoss = newDoorArea * newDoorUValue;
var totalFabricHeatLoss = wallHeatLoss + roofHeatLoss + floorHeatLoss + windowHeatLoss + doorHeatLoss;
// 2. Estimate Annual Heating Demand (kWh/year)
// Simplified calculation: (Total Fabric Heat Loss * HDD * 24 hours/day * 3600 seconds/hour) / (3.6 * 10^6 J/kWh) / (Heating Efficiency / 100)
// Note: This is a very simplified model. A full SAP calculation includes air changes, internal gains, solar gains, thermal bridging, etc.
var annualHeatLossJoules = totalFabricHeatLoss * heatingDegreeDays * 24 * 3600;
var annualHeatLossKWh = annualHeatLossJoules / (3.6 * Math.pow(10, 6));
var annualHeatingDemand = annualHeatLossKWh / (heatingEfficiency / 100);
// 3. U-value Compliance Check
var complianceSummary = '
';
var overallCompliance = true;
function checkUValue(name, value, target) {
var status = (value <= target) ? 'Pass' : 'Fail';
if (value > target) overallCompliance = false;
return '
' + name + ': ' + status + ' (' + value.toFixed(2) + ' W/m²K vs Target ' + target.toFixed(2) + ' W/m²K)
U-value Compliance Check (against typical UK Building Regulations Part L1B targets):
';
resultHtml += complianceSummary;
resultHtml += 'Note: This is a simplified estimate. A full SAP calculation considers many more factors like air tightness, thermal bridging, orientation, internal gains, and specific heating system details. Always consult a qualified SAP assessor for official compliance.';
document.getElementById('result').innerHTML = resultHtml;
}
Understanding SAP Calculations for Extensions
When planning an extension to your home, you'll likely encounter the term "SAP calculation." SAP stands for Standard Assessment Procedure, and it's the methodology used in the UK to assess and compare the energy performance of dwellings. For extensions, SAP calculations are crucial for demonstrating compliance with Part L (Conservation of Fuel and Power) of the Building Regulations.
Why are SAP Calculations Important for Extensions?
Building Regulations Compliance: The primary reason is to ensure your extension meets the minimum energy efficiency standards set by law. Without a compliant SAP calculation, your local authority may not sign off on your project, potentially causing delays or requiring costly remedial work.
Energy Efficiency: A well-designed extension, guided by SAP principles, will be more energy-efficient. This means less heat loss in winter, less overheating in summer, and ultimately, lower energy bills for you.
Comfort: Energy-efficient extensions are more comfortable to live in, with fewer cold spots and more stable internal temperatures.
Environmental Impact: By reducing energy consumption, you're also lowering your carbon footprint, contributing to a more sustainable environment.
Key Factors in an Extension SAP Calculation
While a full SAP calculation is complex, involving many data points about the entire dwelling, for an extension, the focus is heavily on the new elements. Key factors include:
U-values: This is a measure of how easily heat can pass through a material or building element (walls, roof, floor, windows, doors). A lower U-value indicates better insulation and less heat loss. Building Regulations set maximum allowable U-values for new construction and extensions.
Areas of Elements: The size of your new walls, roof, floor, windows, and doors directly impacts the total heat loss or gain.
Thermal Bridging: These are areas where insulation is discontinuous, such as around window frames or at wall-floor junctions, leading to localized heat loss. While complex to calculate precisely in a simple tool, good design minimizes these.
Air Permeability (Air Tightness): How much uncontrolled air leaks in and out of the extension. A tighter building envelope reduces heat loss.
Heating System: The efficiency of the heating system serving the extension (which is often the existing system of the main dwelling) plays a role in overall energy consumption.
Orientation and Shading: How the extension is positioned relative to the sun can affect solar gains and potential overheating.
How This Calculator Works (Simplified Approach)
Our Extension SAP Performance Estimator provides a simplified assessment by focusing on the fabric heat loss of your proposed extension. It takes into account the areas and U-values of your new walls, roof, floor, windows, and doors. It then estimates the total fabric heat loss and an approximate annual heating demand based on typical UK heating degree days and your heating system's efficiency.
Crucially, it also performs a basic compliance check against common U-value targets specified in UK Building Regulations Part L1B for extensions. This helps you quickly see if your proposed insulation levels are likely to meet the required standards.
Important Disclaimer
This calculator is designed for informational purposes only and provides a simplified estimate. It is NOT a substitute for a formal SAP calculation carried out by a qualified and accredited SAP assessor. A professional SAP assessor will conduct a much more detailed analysis, considering all aspects of your specific project and location, to produce the official documentation required for Building Regulations approval.
Always consult with a certified SAP assessor and your local building control department early in your extension planning process to ensure full compliance and optimal energy performance.