Deep retrofit of a 1960s detached house in Harpenden and the addition of a small rear extension. This project is aiming to meet the EnerPHit Plus (Passivhaus) retrofit criteria as well as going 'gas free' by using an air source heat pump and harnessing renewable energy with an entire roof pitch dedicated to solar pv. The existing brickwork walls have been externally insulated using wood fibreboard and finished with a painted render and the curtain walls were removed and replaced with new highly insulated timber I beam walls with cellulose insulation and clad in western red cedar. The rear single storey extension was also constructed in this way and finished with either render or timber cladding. The existing ground floor was also removed and replaced with a new highly insulated slab with underfloor heating.