Whole-House Retrofit Planner

The golden rule: fabric first, systems last

Almost every expensive retrofit mistake is a sequencing mistake. The household that installs a heat pump in a draughty, uninsulated house gets a heat pump sized for the draughts — bigger, pricier, and running harder than it should. The household that insulates first, then sizes the heat pump for the improved building, buys a smaller unit, pays less to run it, and is comfortable in both cases. The same logic runs through the whole plan: draught-proof before you upsize radiators, insulate the loft before you quote solar (a cooler, tighter house changes nothing about the panels, but the discipline of order keeps budgets honest), and always, always do the sub-£500 quick wins before anything that needs scaffolding.

This planner enforces that order with three phases. Phase A — quick wins: LED lighting, the cylinder jacket, draught-proofing, standby control, smart heating controls. Every item costs under £500, most pay back in under two years, all are DIY-able or single-visit jobs. Phase B — fabric: loft insulation, cavity wall fill, glazing. The building envelope: slower paybacks, bigger absolute savings, and the precondition for everything in Phase C. Phase C — systems: solar PV, heat pump, battery. Sized for the improved fabric, quoted after the fabric work is done, never before. Skip a phase and you will pay for it twice — usually in the Phase C quote.

Phase A: the £525 weekend

Run the planner with the defaults and Phase A totals about £525 for roughly £590 a year back — a blended payback under a year. The items, with the numbers this site's calculators produce:

• LED lighting (£80 / £231/yr): the fastest payback in the home. Twenty halogen downlights to LEDs, four months to break even.
• Cylinder jacket (£35 / £85/yr): an 80mm jacket on a bare cylinder. Five months payback, then £85 a year for doing nothing.
• Draught-proofing (£150 / £70/yr): doors, windows, chimney balloon, floorboards. Two-year payback plus the comfort dividend the £ figure misses.
• Standby control (£40 / £44/yr): smart plugs on the entertainment stack. Under a year.
• Smart thermostat + TRVs (£220 / £162/yr): zoned control for the out-all-day household. About sixteen months.

Do all five in a month and the house is already £590 a year cheaper to run before a single installer has been quoted. That is not the whole retrofit — it is the down payment that funds the psychology of the rest. Households that bank quick wins finish big projects; households that start with a £12,000 quote often finish with nothing.

Phase B: the building envelope

Phase B is where the real money moves and where the cowboys live, so it gets the most diligence. Loft insulation to 270mm (£500 / £300/yr) is the best fabric value in Britain — top up from nothing and it pays back in under two years. Cavity wall fill (£1,000 / £350/yr) is excellent if the walls are suitable — run the Cavity Wall Suitability Checker first, because fill in the wrong wall causes damp that costs thousands to remediate. Together they are the planner's default £1,500 for £650 a year, and they are also what makes Phase C cheaper: every kWh of heat loss you eliminate is heat-pump capacity you don't buy.

Double glazing (£5,000 / £165/yr) sits in Phase B on comfort grounds, with the honest flag this site always flies: the payback is measured in decades. Do it for warmth, noise and condensation, in the rooms you live in, and never let a salesman sell it as an investment. Solid-wall insulation (£8,000–£15,000) is the big-ticket fabric item for pre-1920 homes — transformative, disruptive, and worth pricing once the quick fabric is done. The planner leaves it unticked by default for cavity-wall homes; tick it if your walls are solid.

Phase C: systems sized for the improved fabric

Only now — with the quick wins banked and the fabric done — do you quote the big systems, because only now do you know what size you need. Solar PV 4kWp (£6,400 / £605/yr) is the default Phase C anchor: 10.6-year payback at current prices, 25-year asset, and the export income keeps working after payback. The heat pump (£4,500 net of the £7,500 BUS grant) carries this site's standing honesty warning: at a 24p/6p electricity-to-gas price ratio, a heat pump can cost more to run than the gas boiler it replaces (the calculator shows −£56 a year on the standard test case). It still cuts carbon ~1.6 tonnes a year, and on cheaper overnight tariffs the running-cost maths improves — but buy it for carbon and comfort, not for a bill saving that the arithmetic doesn't support.

The battery (£5,000 / £242/yr) is unticked by default and stays that way for most homes: twenty-year paybacks are not investments, they're lifestyle purchases (backup power, outage resilience). Tick it if you have frequent outages or are on a steep time-of-use tariff where the arbitrage works. The planner's job is to show you these numbers side by side so the sequencing decision is arithmetic, not salesmanship.

Budgeting: the £15,000 example

The spec test case — 1970s semi, EPC D, £15,000 budget — is the modal British retrofit, so walk through it. Phase A: £525 out, £592/yr back. Phase B (loft + cavity): £1,500 out, £650/yr back. Phase C (solar 4kWp): £6,400 out, £605/yr back. Cumulative: £8,425 spent, £1,847 a year saved, ~2.5 tonnes of CO₂ cut — comfortably inside the £15,000 budget with £6,575 in hand for the heat-pump decision, the glazing, or the contingency every old house demands. The blended payback of the whole plan is under five years, and the house that emerges is warmer, quieter, and worth more.

Two budgeting rules the planner bakes in: keep 15–20% contingency on any phase involving opening up the building (old houses hide surprises — the loft with no felt, the cavity full of rubble), and never spend the whole budget in Phase C on day one. Prices fall, grants change, and the fabric work you do this year makes next year's system quotes smaller. The planner's budget verdict flags overspend per plan; treat "within budget" as the start of the conversation, not the end.

Getting it done: installers and sequencing in practice

The plan is only as good as the people who execute it. Fabric measures: use certified installers — CIGA for cavity wall, MCS for heat pumps and solar, FENSA for glazing. Get three quotes for anything over £1,000; the spread will shock you (30–50% is normal). Sequence the disruptive work together where you can — scaffolding once for solar + external wall insulation, not twice — and schedule heating-system swaps for spring/summer when you're not depending on the system. Tell each tradesperson what phase you're in: the solar installer who knows the loft is being done next month sizes differently from the one who assumes today's heat loss.

And keep the paperwork. EPC assessments, MCS certificates, guarantees and grant approvals all feed the home's value and any future sale — a documented retrofit with certificates is worth more than an identical undocumented one, because the buyer's surveyor can verify it. The printable summary (browser print from the results panel) is your starting file: plan, costs, savings, CO₂, dates. In January this planner becomes a fuller product; for now, it is the most honest page in the retrofit internet — including about the measures that don't pay.