Extension advice- builder quotes only using building reg plans rather than planning perm plans?

[sugarhoops]

Hiya,

Planning permission went in a few weeks ago, so decision in about 6wks time. Want to do something practical in the meantime whilst we wait, so I thought about getting builders quotes for the work, but our architect said that builders need the building reg plans (which he draws up after planning permission has been granted), and that they could only give a ball park quote against planning perm plans, but nothing definite.

He's the professional so I am assuming correct, but just wondered if anyone has gone ahead and asked for ballpark figures from builders using planning permission plans & not building regs? And were they wildly out once building regs done and builder reassesed?

Your architect is correct. It depends on what you mean by "wildly" The ballpark we had in our head (having spoken to some builders) was about 60% of what the final bill will end up being. This was mainly to do with the way the house is supported (we removed the back of the house and had a relatively complex support structure in place which added a fair few thousand to the build). There were other things too (building regs insited on wider footings on a certain part of the build) which would not have been apparent when just looking at the planning consent drawings.

That said, most of us do a back of a napkin calculation before we even embark even if it is widly optimistic

Your architect should have some experience of similar projects and build costs. There is also a crude m3 cost you can use and increase the estimate if you have particular problems to overcome or you are using expensive materials. This is widely used for insurance claims. Builders won't want to quote on planning drawings but an experienced architect should be able to give you a ball park figure because they will have some idea of the building's spec, eg bricks or blocks, foundations, roof tiles, beams, Eco considerations, drainage etc, if not the finishes you want, eg quality of windows, floors, etc. You should also factor in a sum to finish the garden around the extension or you will have a muddy mess surrounding your house.

We're at this stage. You'll probably find local builders will start pitching their business to you as they keep an eye on Planning and BE submissions. We did invite one over but he could only 'estimate' against the plans and needed the BW plans to 'quote'.
I spend my time at the moment on online 3D planning tools (and have actually completely rejigged what we wanted from the extension because of it! Oopps!)

I would say he is correct. Our architect told us to estimate between £1,100-£1,500 per sq m. Builders we have had round have used the building regs to estimate and in particular the engineers report as we need an RSJ. You could use your time to write up the spec e.g. do you want builders to quote for painting and supplying and laying flooring too? do you want standard fittings e.g. radiators or something different, what about any extra joinery e.g. cupboards built in, do you want to include external landscaping works too? what about lighting? In my experience this will help you compare quotes effectively.

Another word of advice - be prepared to see lots of builders but only get a few quotes back - ask about 6-12 to get about 4 quotes back.
Finally in my experience to date, estimates will tend to be higher than you think they will!