House extension: mechanical specifications on plan

In residential construction, mechanical specifications on plan predominantly refer to the plumbing, heating and ventilation provisions that will be made for your new extension. On your house extension plans you should expect to see the technical detail on a number of items, let’s take a look at the main five.

Thinking through the drainage and first-fix connection detail between your existing house and the extended area is important. The location of the underground and overground soil stack run can significantly impact the extension’s layout, affecting it’s footprint, foundation and configuration of internal spaces. If the stack gets in the way of what you hope to achieve, then plans will either need adjusting to accommodate it, or an engineered work-around contrived.  

An assessment of the existing heating system should be carried out to baseline its capacity to serve the extended footprint for both heating and hot water provision. If it lacks sufficient oomph then an upgrade will be needed. This might be a simple boiler upgrade, or something more involved such as a whole new heating and hot water system, which could interfere with your desired internal layout. In addition, the UK Government intend to ban the installation of gas boilers in homes by the mid 2030s[1], so it is becoming increasingly important to consider the installation of greener systems such as air and ground source heat pumps, to renovation project.

Linked to this, mechanical specifications on plan should account for adequate radiator sizing and positioning. A room’s heat load requirement should be calculated and an optimally-sized radiator selected to serve that room. The plans should position these radiators such that they do not interfere with the amenity or enjoyment of the new space. Let’s bust the myth that radiators or electric heaters need to be positional under or adjacent to a window if they are to effectively heat a room, because they actually heat a space using circulatory convention and allow for an even distribution of heat through a geometrically uniform area. With underfloor heating, pipes need only be run under areas of pedestrian traffic, and not under dead areas like cupboards. They should also take account for the space needed to house the manifold.

Climate change has made +30°c summers in Britain and Europe common, so if air conditioning systems are to be installed, the working drawings would benefit from showing the location of the internal and external units. Indeed, passive cooling systems native to homes in the tropics are a steadily increasing trend, though perhaps beyond the remit of this book.

Lastly, a supplementary note on any ancillary mechanical works that may be required. In some instances, in may be required to re-route the incoming water mains or gas supply either wholly or partially in order to realise the desired layout. This should be reflected in the working drawings, as should any design changes required to house the machinery of water softeners, booster pumps or accumulators for mist systems.

A majority of the mechanical specifications on plan can be worked through (at least at a high level) in advance of the build phase of the project. Where applicable it is advised to engage the relevant heating or cooling specialists at this stage so you can work-in any necessary operational changes on plan to facilitate their supply.

If you would like some free planning advice, get in touch with us.