Mad to consider a massive house extension?

[Figure17a]

We have a 4 bed detached with a decent sized garden.

The kitchen runs across most of the back of the house and is a good size, but a fairly narrow rectangle. There's a small dining room next door. Upstairs the back has bathroom and two smallest bedrooms.

I have this plan to extend 2 stories by 3m (which I think can be done without planning.) which would give a massive square family room/kitchen, and double the size of the two small bedrooms, dining room and bathroom. Hopefully a bit could be taken from larger of two bedroom for an additional bathroom (only one plus downstairs loo atm)

So madness to consider such a big project, or great way to make our house work the way we want it without having to move?

Which brings most hassle, a project like this or moving? How do I get a ball park figure on the cost and time taken?

Do I start by finding a builder or an architect?

Any advice? Even if it's don't!

We did something very similar to a house we owned. We started of wanting to extend the downstairs and the builder said that the main cost was in the roof and if they took the wall up a few feet further we could have two more bedrooms as well (with dormers) at very little extra cost.

We did this and loved it - lots of lovely space. We knew the builder was an honest chap (patient of OH) and went to France for 3 weeks - all done when we got back!

Well, that's encouraging, although find it hard to believe building half a house including a kitchen and two bathrooms will be over in three weeks!

I have a builder in mind and we've left him in the house before, I'd certainly plan to be out as much as possible.

I honestly don't know where to start though. Do we get plans before approaching a builder or does the builder advise re plans required? Does the builder do the kitchen etc or do we need separate kitchen and bathroom companies?

How can I get a rough idea of if it's even possible financially without wasting builder/architect's time?

£1500 per m2 of new build and an allowance for any reconfiguration of existing space plus kitchen cost (~£15k) and any bifold type doors costs (~£10k for 6m) would give you a ball park figure.

Builder can put in allowance for kitchen fit(~£1k) but better to have kitchen company do it.

Thanks Outandln. Is that £1500 per m2 of the footprint or £1500 per m2 of floor space (i.e upstairs + downstairs?)

Per m2 of actual space - so 3mx8m extension double height = 47m2 = £72k

You are looking at £100k minimum i would say.

Yes, that's the figure I plucked out of no-where Outandln. If we spent £100k on moving, we'd probably get an extra bathroom and maybe a utility but I don't think we'd get much more than what we have now in terms of space.

I may be wrong but I think you can only build a single storey extension under permitted development rights so would need planning permission. In any case an informal chat with your local authority's planning department may be a good place to start.

I've checked the planning rules and it seems to me that if the extension is on the rear, you can go out 4m on the ground floor and 3m on two floors, provided the roofline is no higher and the extension ends more than 7m from the back fence. Any side windows need frosted glass.

One of our family members did something similar. Took very much longer than 3 weeks and they had to move out for a bit. Looks amazing though.

100k + is right. To start, I would ask locally for recommendations of a builder or building company, who will have contacts with architects, plumbers, kitchen fitters etc. They should be able to give you very broad guidelines around cost, time scales etc. If it sounds like a goer, then engage an architect or structural engineer and go from there.