The deep-retrofit decision tree: what to do, in what order, with which budget

The most common question I get from friends about retrofit is the most reasonable one: if I have £X to spend, what should I do? The honest answer depends on the house, but the structural decision tree is more universal than people realise. There is a sensible sequence for the typical UK building stock, and it scales reasonably consistently across budget brackets — what's the £5k version of fabric-first, what's the £25k version, what's the £75k version that actually adds up to a deep retrofit.

This piece is the decision tree I'd actually use, with three budget paths laid out side-by-side: a tactical-fixes budget, a moderate-retrofit budget, and a deep-retrofit budget. The principle is the same throughout — fabric first, controls second, generation third, with sequencing dictated by building physics and grant availability — but the scope at each budget tier is genuinely different, and being honest about that helps avoid the false comparisons that wreck a lot of retrofit planning.

The numbers below are real 2026 UK costs for a typical 3-bed semi at EPC D. Bigger houses scale roughly linearly; smaller houses, slightly less. A bungalow with high ground-floor exposure or a top-floor flat with high roof exposure have different priorities, but the framework holds.

The principle: fabric, controls, generation, in that order

The structural rule that survives every budget bracket is: do the fabric work first, the controls and ventilation second, and the generation and storage last. The reason isn't ideological. It's that the heat-loss reduction at each stage shrinks the requirement for the next, so doing them in this order means you end up with a smaller (cheaper) heating system and a more right-sized renewable system at the end.

Specifically: fabric upgrades reduce the design heat-loss day requirement, which reduces the kW size of the heat pump or boiler you need. Smaller systems are cheaper, simpler, and run more efficiently in the part-load conditions they spend most of their time in. Doing the heat pump first means you've sized for the old, leaky building, and end up with an over-spec system that part-loads badly for the rest of its life.

Controls and ventilation come second because they tune the fabric-improved building to the way it's actually being used. Without controls, the fabric savings are diluted by overheating; without ventilation, the airtightness work creates moisture problems.

Generation comes last because (a) you can size it correctly only once you know the actual demand profile of the post-retrofit house, and (b) the grant schemes generally pay you for it after the fact. There's no economic reason to front-load PV before the rest of the work has been done.

Path A: the £5,000 tactical retrofit

This is the budget that lots of households actually have. It won't transform the house. It will materially reduce the bills and fix the worst problems. The right scope for a typical EPC D 3-bed semi:

1. Loft insulation top-up to 300mm — £400–£900
2. Cavity wall insulation (if cavity-walled and not done) — £500–£1,500. Free under ECO4 if you're eligible.
3. Hot water cylinder insulation jacket if cylinder is older and uninsulated — £40
4. Draught sealing, door brushes, sash window sealing — £300–£800
5. Smart thermostat with TRVs — £350–£700 fitted
6. Loft hatch insulation, basic pipe lagging — £100–£200
7. Hygrometers in each room and an audit of moisture sources — £40 of kit, no install cost

Total: roughly £1,800–£4,200 cash spent (the rest of the budget allows contingency or modest upgrade). EPC uplift typically half a band (D to mid-C). Heating bill reduction 12–22%. Payback 3–6 years. The biggest single saver in this list is usually the cavity wall insulation if it hasn't been done yet.

Path B: the £25,000 moderate retrofit

The bracket where the conversation gets more interesting and the trade-offs start to bite. The right scope:

1. Everything from Path A — £4,200
2. Suspended floor insulation if applicable — £1,500–£3,500
3. Window upgrades on worst-performing units (typically 4–6 windows) — £3,000–£6,000
4. Improved ventilation strategy — extract fan upgrades, possibly PIV — £400–£1,200
5. Air-source heat pump install with cylinder and radiator upgrades, after BUS grant — £4,500–£8,500 net of £7,500 BUS
6. 4 kWp solar PV array (no battery) — £4,800–£6,500
7. Move to time-of-use tariff and configure controls — labour in the install, maybe £200 of additional kit

Total: £18,400–£29,900 cash spent. This is the genuine 'moderate retrofit' bracket. EPC moves to high B or low A. Heating bill reduction 35–55% versus pre-retrofit. Payback period for the package as a whole 9–14 years, but individual measures vary widely (cavity wall pays back in 2 years, windows in 20+).

What you don't do at this budget: external wall insulation, three-phase supply, MVHR. Those are deep-retrofit measures.

Path C: the £75,000 deep retrofit

The serious budget. This is where the retrofit becomes a genuine deep retrofit with EPC A target and substantial fabric work. Right scope:

1. Everything from Path B — £25,000
2. External or internal wall insulation on solid walls (most Victorian/Edwardian terraces and some interwar) — £10,000–£25,000
3. Full window replacement to A-rated glazing — £8,000–£12,000
4. MVHR system with full ductwork if airtightness is being pushed below 5 m³/h/m² — £8,000–£14,000
5. Roof insulation to passive standard if there's a loft conversion or warm-roof opportunity — £4,000–£10,000
6. Heat pump upsized or upgraded to GSHP if the building is now low-demand enough that the marginal cost is justified — £8,000–£18,000 net
7. 10 kWp PV + 15 kWh battery — £15,500–£23,500
8. EV charger with solar diversion and time-of-use integration — £1,200–£1,800
9. Full smart-home integration using the longevity principles — £2,000–£5,000

Total: £71,500–£110,000+ cash spent over 2–4 years. This is the bracket where PAS 2035 assessments become essential, where a Retrofit Coordinator role is worth paying for, and where the project is genuinely engineered rather than specified item-by-item.

EPC moves to A. Heating bills drop 70–85%. Payback at this scale is 15–22 years on running cost alone, but the resale-value uplift, comfort improvement, and emissions reduction are doing as much of the justification as the energy bill.

Grants and where they fit

The UK grant landscape in 2026:

• BUS (Boiler Upgrade Scheme): £7,500 for ASHP/GSHP install via MCS. Available to owner-occupiers of existing homes. Can't be combined with ECO4 for the same measure.
• ECO4: Income- or property-eligibility-based fabric and heating measures. Substantial when it applies; complex eligibility. Worth checking even if you think you don't qualify, because the LA Flex extension captures households the standard scheme doesn't.
• GBIS (Great British Insulation Scheme): Insulation-only, typically loft and cavity, for households on certain criteria. Worth checking.
• Local authority discretionary funding: Varies dramatically by council. Worth a 10-minute search of your council's website.
• Zero VAT on residential renewable installs (PV, batteries, heat pumps). Already priced into 2026 quotes but worth confirming.

The pattern: grants typically cover the fabric-first measures (most generous for low-income households) and the heat pump (BUS for everyone). They rarely cover the controls, ventilation, or generation. Plan for that split.

When to use a Retrofit Coordinator vs going DIY

For Path A, you can run it yourself. Get three quotes per measure, supervise the work, sequence the order yourself.

For Path B, it's a marginal call. Some households can handle it; others benefit from a Retrofit Assessor or independent advisor (£400–£1,200 fee) who produces a sequenced plan and reviews quotes.

For Path C, hire a Retrofit Coordinator. PAS 2035 actually requires it for ECO4-funded deep retrofits. Even outside grant funding, the Coordinator function — heat-loss modelling, moisture-risk assessment, sequencing, quality assurance — is worth £3,000–£8,000 on a £75k+ project to avoid the kind of physics mistakes that compound expensively. The biggest deep-retrofit failures I've seen in the UK have been ones where no Coordinator was involved.

The choice is partly about your own capacity to project-manage and partly about whether you trust the installers' sequencing instincts. For most households at deep-retrofit scale, the Coordinator earns their fee.

Common decision-tree mistakes

The mistakes I see most often, regardless of budget bracket:

1. Solar PV before insulation. Tempting because solar feels exciting and grants apply. But generation on a leaky building is paying premium rates to heat outside. Insulate first.
2. Heat pump on existing radiators with no fabric work. Almost always produces a low-SPF install that runs expensively and tarnishes the owner's view of heat pumps in general.
3. Internal wall insulation without a moisture strategy. Creates interstitial condensation. Always pair IWI with airflow design.
4. Window replacement before airtightness audit. The seal around the window matters more than the glazing; doing windows first means you don't know what your remaining airtightness problems are.
5. Battery before tariff change. Battery economics are tariff-driven. On a flat tariff the battery doesn't pay back.
6. MVHR in a moderately airtight house. MVHR is for genuinely tight homes (1–3 m³/h/m²). In a 5–7 m³/h/m² house it's over-spec; trickle vents and good extract fans do the job for a fraction of the cost.

The decision tree in one page

Stripped to its essentials:

1. What's the budget? £5k → Path A. £25k → Path B. £75k+ → Path C.
2. Is the building dry? If no, fix the roof and gutters first. Independent of budget.
3. Is loft insulated to 300mm? If no, do this regardless of budget. Cheapest, fastest payback.
4. Cavity wall, uninsulated? Fill it. Fast payback.
5. Solid wall? Path C territory. Paths A/B don't address it.
6. Suspended floor, accessible? Insulate. Underrated win.
7. Old, single-glazed, or failing windows? Path B+.
8. Old or oversized boiler / oil/LPG? Heat pump becomes the right answer once fabric is done. Path B+ with BUS grant.
9. Decent south-facing roof? Solar PV, sized at 4–10 kWp depending on consumption.
10. Time-of-use tariff? Move to one. Battery becomes worth it with the right tariff.
11. Driving an EV or planning to? Smart charger with solar diversion at install time.

That's the tree. The sequence above is reasonable for almost any UK home. The only question is which path budget supports.

Deep retrofit isn't a list of measures. It's a structured decision about sequence, budget, and physics. The right answer for any given household is the right path (A, B, or C), executed in the right order, with the right grant capture, with professional oversight where the budget warrants it. None of the three paths is wrong — each is the appropriate response to the budget the household actually has.

The thing I'd most warn against is the off-piste retrofit: a heat pump here, a bit of solar there, a window replacement next year, no integrated plan. That route produces a lower outcome at higher total cost, because each measure was sized for the building as it was rather than as it'll be after the next measure. The decision tree exists to force the integration.

Whichever path applies, the principle is constant: fabric first, controls second, generation last. The technology is ready. The grant landscape is workable. The sequencing is the part that's still in your hands.