Hounslow is really two boroughs for planning purposes. The eastern end — Chiswick, Strand on the Green, Brentford and Isleworth — is dense, historic and heavily conservation-protected. Head west into Heston, Cranford, Feltham and Hanworth and you're in largely inter-war and post-war suburbia with very little heritage constraint. Which half you live in changes the answer to almost every question below.
The council currently lists 29 conservation areas, concentrated overwhelmingly in the east of the borough. Only two carry Article 4 directions affecting external alterations: Bedford Park and Gunnersbury Park, both in Chiswick — covered in detail in the Chiswick area guide. That's a low figure by inner-London standards — Islington, by contrast, has 40.
Hounslow also operates a borough-wide Article 4 direction covering conversion of houses to small HMOs. It has no bearing on solar panels or heat pumps, but it's the reason some address-checking tools will flag an Article 4 restriction on every property in the borough. Ignore that flag unless you're converting to an HMO.
The east is dominated by Edwardian villas in Chiswick and Turnham Green and Georgian stock around Isleworth and Old Chiswick. These are typically narrow-plotted terraces and semis, and the practical constraint is almost never planning — it's the 1-metre boundary rule for an air source heat pump. On a Chiswick terrace with a side return under two metres wide, siting a unit that satisfies both the boundary distance and sensible airflow clearance is genuinely tight, and it's the single most common reason a survey comes back negative here.
West Hounslow is a different proposition entirely. The 1930s semi-detached stock around Feltham, Hanworth and Heston typically has proper side access and rear gardens, generous unshaded roof pitches, and cavity walls that may well be uninsulated — which makes it some of the best value retrofit territory in west London. If your loft and cavities aren't done, start there before quoting for a heat pump.
Where you do need a planning application, Hounslow determines householder applications at an approval rate around 66% — well below the national average of roughly 87%. That doesn't make approval unlikely, but it does mean a marginal or poorly documented application is more likely to fail here than in most boroughs. If you're in Bedford Park or Gunnersbury Park, pre-application advice is worth the fee.
Roughly a third of the borough is Green Belt or Metropolitan Open Land, which rarely affects roof-mounted solar but can complicate ground-mounted arrays or ground source heat pump trenching on larger plots.
For costs, grants and how the national rules work, see the heat pump guide, solar guide, air conditioning guide and insulation guide.